'Snape!' ejaculated Slughorn, who looked the most shaken, pale and sweating. 'Snape! I taught him! I thought I knew him!' (Half-Blood Prince, Chapter 29)

Considering the long and illustrious list of future mass murderers that Slughorn taught, knew, and was quite friendly with:

  • Tom Riddle, aka Lord Voldemort

  • Death Eaters (We see Lestrange and Avery in Slughorn's memory, socializing with him among other Riddle's friends all of whom became Death Eaters later)

  • Or, for that matter, Snape himself. He DID turn Death Eater right after graduating school.

... what would cause Slughorn to somehow assume that teaching and having knowing someone would be effective at figuring out they aren't a killer?

  • 1
    You assume he was surprised that the others were murderers. Based on the pensieve memory, I doubt very much he was surprised that Tom turned out the way he did. Commented Dec 3, 2014 at 22:00
  • 5
    Weeeeeeeeell, most people are shocked or surprised when someone they know commits a homicide, much less more than one. When you think you know a person and they turn out to have criminal elements, it's shocking. Why wouldn't he be surprised? :) Commented Dec 3, 2014 at 22:04
  • 17
    Completely unrelated, but I can't believe Slughorn ejaculated with all those children around.... I really hope it means something else in the UK. Commented Dec 3, 2014 at 22:07
  • 16
    @DaveJohnson It's a pretty old usage of the word, but it's similar in meaning to "exclaimed". Commented Dec 3, 2014 at 22:22
  • 1
    @DaveJohnson I remember laughing childishly at that sentence when I first read that book.
    – March Ho
    Commented Dec 25, 2014 at 21:35

6 Answers 6


I think Slughorn's surprise is based on the fact that Snape had something that all his other spoiled students don't have: Dumbledore's trust.

Slughorn trusts Dumbledore, and Dumbledore assures that Snape is their ally. Hence, Slughorn trusts Snape.

The other students, like Lucius, Bellatrix and Tom, don't have Dumbledore's trust. Then, Slughorn has no reason to trust them either.

So ultimately, Slughorn is not surprised that Lucius and Bellatrix are out there killing people, because he has no reason to believe they're good. But on the other hand, he does believe Snape is good because of Dumbledore... Then Snape kills Dumbledore, and I think this is the reason Slughorn is so surprised.

Now, the above post only addresses why is Slughorn surprised. It does not address why did Slughorn decide to choose these words:

I taught him! I thought I knew him!

Rather than

I trusted him!

I personally believe that there's no special meaning behind these words. It's just Slughorn agitated by the news of Dumbledore's murder - just like everyone else.

  • 6
    I think the important exclamation here is "I thought I knew him!". Slughorn also taught the others as well but he was not suprised that they turned out to be murderers. It is just that his judgement failed him in Snapes case. Commented Dec 4, 2014 at 8:45
  • 20
    Should we really trust Voldemort's opinion on this matter? Commented Dec 4, 2014 at 9:40
  • 10
    @AndrewMartin Why not? Voldemort clearly knows Slughorn better than the Muggles around here.
    – Geobits
    Commented Dec 4, 2014 at 15:19
  • 2
    @Geobits: Good point actually! Commented Dec 4, 2014 at 15:20
  • 1
    I think saying "I taught him!" implies the length of time he's known Snape. It hasn't just been since Slughorn returned to Hogwarts, but rather since Snape was just a child. It sets up the length of time the trust has been established.
    – user31178
    Commented Dec 5, 2014 at 3:34

I think this section is a bit of a red herring. Slughorn's a bit of a narcissist - perhaps not someone who formally suffers from the disorder, but certainly someone who only wants to associate with the brightest and best (thus his special invitation-only parties). But, like many narcissists, he has a deluded sense of his own importance, and how he appears to others; in this sentence he focuses on himself and his relationship with Snape - "I taught him - I thought I knew him" - whereas other, less self-absorbed people might just focus on Snape's apparent crimes - e.g. "How could he do something so awful?"

I think he is shocked and shaken not because he assumes teaching / knowing someone means he can predict who will and won't be a murderer, but because Snape's act undermines his believed understanding of Snape's character, and also suggests that he may have gone wrong somewhere, as Snape's teacher.

  • 1
    You're not addressing my main point of the question. He should EXPECT such a turn of events, or at least not be surprised, since (1) He had the same thing with RIddle. (2) Then Riddle's Death Eater friends (3) Then Snape's class (Mulciber etc..)... and (4) For that matter, Snape turned Death Eater upon graduation! Commented Dec 3, 2014 at 22:45
  • 3
    i think bassed on what ive read about slughorn, he was surprised EVERY time this happened, despite the fact that its happened before, and snape was a highly suspect individual, i would expect this reaction every single time someone in his club turned out bad. I think Slughorn takes it as a personal blow everytime against his ego.
    – Himarm
    Commented Dec 3, 2014 at 22:49

The web-of-trust argument is good but raises the question of why he said:

I taught him! I thought I knew him!

Snape was —like Slughorn— a potions expert. He would have done potions to the highest level at Hogwarts, under Slughorn's tutoring. There are a few significant considerations to this:

  • 7 years of curricular contact time is not insignificant.

  • Snape was a Slytherin, the house Slughorn was the head of. The two would have had lots of extra-curricular contact at Hogwarts.

  • Lily Evans was in Snape's class. His best friend and eventual love interest, it would have been easy to see the growing infatuation between the two in the class. Slughorn may have considered somebody so full of love unable of the highest evils.

All combined, it's possible that Slughorn considered him a generally nice person with the same interests. It's easy to assume those sorts of people are similar to you and are similarly incapable of murder.

Even once Snape joined the Death Eaters, to the right sort of person, just being a Deatheater was a positive thing. Their rise was subtle, under the creeds similar to magic is might, and many magicking families (eg the Blacks) were initially supportive of this. Given Slughorn's investment in Voldemort at Hogwarts, it doesn't seem unlikely that he also supported his wider ethos. It's also possible that Slughorn suffered from confirmation bias, blinding him to Voldemort's lesser crimes.

It wasn't until later that the Death Eaters started committing high crimes in public. It was all around this time that the Ministry prosecutions started and Snape was turned by Dumbledore.

It's also possible that Slughorn didn't know about Snape's stint as a Death Eater until it was all over... By which time Dumbledore had all but labelled him as a hero.

(On Riddle's classmates: they weren't Death Eaters at school when Slughorn saw them... They were a group of Slytherins which is how he would have seen them.)


They must have been very close when Severus was a student. We shouldn't forget Snape is the Half-blood Prince who made scribbles in his book when he was doing potions in class. Do we have any other people around who correct book authors at the age of 15? He must have been very Hermione-like back then thus he must have brewed his way into the Slug club. Most likely.

If we turn to the film adaptation (not a very good resource, but didn't Rowling co-author the screenplay? Not sure) Slughorn says at the beginning of Harry's first Potions lesson that

Only once did a student manage to brew a potion of sufficient quality to claim this prize (Felix Felicis)

Who that one student may be? Two possible candidates: Lily and Severus. But I think it's just Severus :) He must have been one of the most favorite students. They could have had 'private' Slug clubs, Severus being that brilliant at Potions, he could potentially live besides his cauldron and Slughorn.

That's why I guess Severus was not just an ordinary student for Slughorn, he was very much special.


Other answers have touched upon this, but I fear that they have gotten somewhat distracted by the details so it is worth stating it succinctly. The question appears to be based on a mistaken premise. Slughorn's surprise that Snape was a murderer was not because Slughorn wouldn't expect any of his students to be murderers. Indeed, as mentioned in the question, several of Slughorn's students were murderers.

Rather, Slughorn's surprise was specifically that Snape became a murderer. This surprised him because he thought he knew Snape. What he thought he knew of Snape was not the type of person who would be a murderer. Slughorn is expressing his surprise that Snape is a murderer despite not appearing like the type of person who would be a murderer. The statement "I taught him" is simply the setup for "I thought I knew him". Slughorn was very familiar with Snape precisely because he had taught him for many years.


I've just reread my answer from almost five years ago and I'm now wondering...

Why did I ever assume that Slughorn is lawful-good?

Throughout his scenes, we see a well spoken and outwardly polite elderly chap, but we also know he is Syltherin and a massive egotist:

  • He surrounds himself with people he thinks will be a success
  • He invests his influence in those people, placing them in his debt
  • He collects on those debts when they mature; when the person is in a position of influence themselves

You could argue this drive for success is his only Slitherine quality, but another explanation is he is just a functioning psychopath. His absolute self-preservative drive, lust for power, ability to completely blank people outside his circle (eg Ron, Arthur), and perhaps an inability to understand others' drives.

Snape! I taught him! I thought I knew him!

If we can consider him with an antisocial personality disorder, I think it's also possible that when he says things like this, its his self-preservation kicking in and saying the things he thinks people need to hear. He seems sincere in his interactions —even drunk* with Hagrid and Harry— but that's how some ASPDs present.

*Important to remember that he was not drinking veritaserum. He could still lie.

  • This seems overly conspiratorial and at odds with his other behaviour. While he's definitely not a good guy, he also never joins Voldemort, is horrified and ashamed of his interaction with Tom about the Horcrux, and does (in the end) fight during the Battle Of Hogwarts when he could have easily fled. As Sirius says, the world is not divided into good people and Death Eaters, and Slug is at least consistently presented as having a conscience, even if he tries to ignore it.
    – DavidS
    Commented Apr 29, 2019 at 9:59
  • I'm not saying he's evil, just suggesting he's more chaotic-neutral than his interactions portray. He is always outwardly horrified, always saying the right thing, but we have no internal monologue to verify his feelings. All his actions indicate power-mongering and self-preservation are his drives, without conscience, not feeling objective goods or evils, allowing him to swing either way. I do also think his Battle for Hogwarts actions can be rationalised with this too.
    – Oli
    Commented Apr 29, 2019 at 10:30
  • That all said, this is supposed to be an alternative viewpoint. [As you quote] good/evil is not a binary quality, and we have very little written substance to explain why Slughorn (etc) does the stuff he does. The greys in life are full of mistakes, personality disorders, historical context but the other answers (mine included) ignore the possibility of that, in favour of tidy, rational, lawful-good explanations.
    – Oli
    Commented Apr 29, 2019 at 10:35
  • that seems a little contradictory - you start with "he's pretending to care, but his actions say otherwise" and then immediately follow up with "when he does good actions it's for selfish reasons, not because he cares". By that logic can't we argue that any character (besides Harry) is a secret psychopath who only ever pretends to care? But yes fair enough, as a potential/alternative explanation I suppose it is possible.
    – DavidS
    Commented Apr 29, 2019 at 10:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.