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Fan consensus is that if Hermione's claim in chapter 9 of Deathly Hallows is not a continuity error, then it means that she was wiping the two Death Eaters' recent memories rather than modifying them. The relevant block of text, conveniently using the word "wiping" is as follows:

"We just need to wipe their memories," said Harry. "It’s better like that, it’ll throw them off the scent. If we killed them it’d be obvious we were here."
"You’re the boss," said Ron, sounding profoundly relieved. "But I’ve never done a Memory Charm."
"Nor have I," said Hermione, "but I know the theory."
She took a deep, calming breath, then pointed her wand at Dolohov’s forehead and said, "Obliviate."
At once, Dolohov’s eyes became unfocused and dreamy.

By taking this action, the trio are making some assumptions that I find questionable:

  1. Rather than use a spell that she is so skilled in that she successfully used it to radically change her parents' memories and life ambitions, it is better for Hermione to use a spell that she has never used before. The idea that she may botch the spell, putting everyone's life at risk either immediately or as soon as the Death Eaters wake up, is not considered.
  2. Wiping the memories of these Death Eaters is somehow a better choice than editing said memories to make them think that they found nothing, found the wrong people, or that they attacked each other.
  3. The Death Eaters, upon waking up and having no idea how they got to where they are, will not suspect that they found their target and lost the battle.
  4. The trio, despite having access to teleportation, extra-dimensional storage, and the ability to reduce objects to a fine powder, are incapable of making it non-obvious that they've killed someone.

So why did they act in this way? Surely, the better move was to modify the Death Eaters' memories rather than wipe them? I can understand point #4 being rejected on moral grounds, but the other three assumptions cause me great confusion.

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  • As seen in book 6, modified memories are fairly easy to detect, even when done by a highly skilled professor (Slughorn). Hermione herself says: "It’s better like that, it’ll throw them off the scent. If we killed them it’d be obvious we were here." -- seems, well...obvious – NKCampbell Nov 30 '20 at 19:49
  • @NKCampbell Strange how whenever these are used for a crime in series, the victim never notices. Hermione's parents definitely never noticed anything after her huge edit. – J. Mini Nov 30 '20 at 21:27
  • Also the Harry & co are on the lam and making it up as they go along. Given more time they might have thought of a better plan, or at least considered the points you make. But they needed to act quickly. – Paul Johnson Dec 1 '20 at 11:55
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We have no idea how memory editing works, but we do know more about Obliviate

Hermione's spell might have been hard

One of the premises of this question is that the spell Hermione used on her parents was easy to do, but actually we don't know how hard it was, or if it required a lot of preparation. We can assume, however, that she had much more time to get it right with her parents, with much less pressure of failure, as compared to the situation with the Death Eaters.

Obliviate is a spell which is not time consuming and does not require the cooperation of the target

This is something we see a lot in Chamber of Secrets, especially when Lockhart tries to enchant Ron and Harry.

Part of Obliviate seems to be that victims are not aware of it

It doesn't seem to be the case that people are simply missing a few hours of their memories. Instead, none of the victims that we meet in canon is aware that they are missing a memory. Depending on how the spell went, they might be confused, but they generally do not seem to question it.

The trio covers their tracks

After performing Obliviate on them, the trio cleans up the cafe and puts the Death Eaters in a booth facing each other, seemingly to make it easier for the memory removal to "fit in":

"Clear up?" said Ron, looking around the partly destroyed cafe. "Why?"
"Don't you think they might wonder what's happened if they wake up and find themselves in a place that looks like it's just been bombed?"
"Oh right, yeah..."
[...]
Once the cafe was restored to its previous condition, they heaved the Death Eaters back into their booth and propped them up facing each other.

(Harry Potter and the Deathly Hallows, Chapter Nine: A Place to Hide)

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  • We definitely know that editing does not require consent. Riddle's two uses tell us that. – J. Mini Dec 5 '20 at 13:37
  • Good point, I will edit it in later – Philipp Dec 5 '20 at 14:17
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Memory modification can be undone

Charms used to modify memories can be resisted and the individual whose memory was modified can regain their original memories. We witness this in Book 4, when Voldemort learned from Bertha Jorkins that Barty Crouch Jr. was alive and with his father even though Barty Crouch Sr. had placed a powerful memory charm on her.

We could have modified her memory? But Memory Charms can be broken by a powerful wizard, as I proved when I questioned her. It would be an insult to her memory not to use the information I extracted from her, Wormtail.” - Voldemort to Wormtail

The Riddle House: Harry Potter and the Goblet of Fire, page 12

Simply modifying their memory would not have been sufficient to "throw them off the scent". A powerful wizard like Voldemort would have easily detected the modification and extracted the necessary information.

Using the Killing Curse would have been traceable

Additionally, the moral dilemma of your fourth point aside, killing the Death Eaters with magic would have actually been traceable. As Dumbledore tells Harry in Book 6:

“How did you know that was there?” Harry asked in astonishment.

“Magic always leaves traces,” said Dumbledore, as the boat hit the bank with a gentle bump, “sometimes very distinctive traces. I taught Tom Riddle. I know his style.”

The Cave: Harry Potter and the Half Blood Prince

While the books don't go into more detail about this, if the magic of a wizard as powerful as Voldemort was traceable, then I think it's reasonable to infer that Voldemort would have very easily been able to identify that the trio were here and had used the Killing Curse.

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  • Who said anything about the Killing Curse? This far in to the series, we haven't seen anyone in the trio cast it. If magical tracking is a concern, one only need use a knife. I also don't see why it's likely that the Death Eaters in question would noticed that their memories have been modified and try to have that fixed. You can say that the Slughorn example shows that modified memories are easy to spot, but Morfin, Hokey, and Hermione's parents are all examples to the contrary, showing that the victims didn't notice the fakes. Still, good job. I didn't expect any answers as plausible as this. – J. Mini Nov 30 '20 at 21:36
  • Also its likely that the death eaters would not want to draw attention to their possible failure, so would agree between themselves not to probe that missing time to closely or mention it to anyone else. – Paul Johnson Dec 1 '20 at 11:54
  • @J.Mini the average witch/wizard may not have noticed but I feel that Voldemort would have been able to identify modified memories. He was able to deduce that Bertha Jorkins memory had been modified even though Barty Crouch Sr. had placed a memory charm on her that was so powerful that it permanently damaged her memory. – Anshuman Dikhit Dec 2 '20 at 0:44
  • @AnshumanDikhit And why would Voldemort look? – J. Mini Dec 2 '20 at 0:58
  • @J.Mini any answer to that would just be pure speculation, so I can't give as plausible a response. My thought process was more along the lines that he CAN detect that the Death Eaters memories have been modified and would undo the modification to figure out why. So better off to just wipe their memory entirely. – Anshuman Dikhit Dec 3 '20 at 4:32

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