Why didn't Quirrell try to kill Ron and Hermione?

Question

Quirrell attempts to assassinate Harry during the Quidditch match in Philospher's Stone.

"But Snape tried to kill me!"
"No, no, no. I tried to kill you. Your friend Miss Granger accidentally knocked me over as she rushed to set fire to Snape at that Quidditch match. She broke my eye contact with you. Another few seconds and I'd have got you off that broom. I'd have managed it before then if Snape hadn't been muttering a counter-curse, trying to save you."
"Snape was trying to save me?"
"Of course," said Quirrell coolly. "Why do you think he wanted to referee your next match? He was trying to make sure I didn't do it again. Funny, really...he needn't have bothered. I couldn't do anything with Dumbledore watching. All the other teachers thought Snape was trying to stop Gryffindor winning, he did make himself unpopular...and what a waste of time, when after all that, I'm going to kill you tonight."
Quirrell snapped his fingers. Ropes sprang out of thin air and wrapped themselves tightly around Harry.
"You're too nosy to live, Potter. Scurrying around the school at Hallowe'en like that, for all I knew you'd seen me coming to look at what was guarding the Stone."
_{(Philosopher's Stone, Chapter 17, The Man with Two Faces).}

Quirrell's stated chain of logic is that:

• Harry had been scurrying around the school at Halloween.
• This made Quirrell think that Harry may have realised that Quirrell wanted the Stone.
• Quirrell concluded that Harry was "too nosy to live".
• Quirrell tried to kill Harry during the Quidditch match.

Yet why was Harry singled out for murdering? If Harry was too nosy to live then surely Ron and Hermione were too? The event which sparked off Quirrell's attempted assassination was Harry being out his dormitory during the Halloween troll attack. Yet Ron and Hermione were also out and about that evening. (Hermione was only there because she wasn't aware of the edict to go back to the dormitories - but Quirrell wouldn't have known this since Hermione lied to McGonagall about her reasons for being there). Surely if Harry being caught fighting the troll made him a target then Ron and Hermione should have been targets too?

Yet Quirrell chose to try and kill Harry on the Quidditch pitch, the one place where he knew the trio couldn't be attacked together. His methods and his language suggest that he wasn't interested in Ron or Hermione.

He wasn't acting out of ignorance. Quirrell knew that Ron and Hermione were with Harry on Halloween because he discovered the trio together with the troll that night, alongside the other teachers.

A moment later, Professor McGonagall had come bursting into the room, closely followed by Snape, with Quirrell bringing up the rear. Quirrell took one look at the troll, let out a faint whimper and sat quickly down on a toilet, clutching his heart.
_{(Philosopher's Stone, Chapter 10, Hallowe'en).}

Since Quirrell knew that Harry, Ron and Hermione had been out-of-bounds together at Halloween why didn't he try and kill all three of them? Why did he only try to kill Harry?

Answer 1

The Quidditch match was a good opportunity to make Harry's death look like an accident

The reason Quirrell made an attempt to kill Harry at the Quidditch match was because it was an opportunity too good to miss - after all, Quidditch is a dangerous and accident prone sport:

‘Er – have the Bludgers ever killed anyone?’ Harry asked, hoping he sounded offhand. ‘Never at Hogwarts. We’ve had a couple of broken jaws but nothing worse than that. _{~ Harry Potter and the Philosopher's Stone - Chapter 10 : Halloween}

So broken bones are pretty common. Wood's answer also seems to imply that, in places other than Hogwarts, there were some Quidditch fatalities.

Harry's broom in his first year, the Nimbus 2000, was a racing broom which (with its quick acceleration) might require skills beyond an average first-year's capabilities. It could be that Quirrell hoped people would think that Harry fell to his death due to his inexperience.

The original plan was probably make it appear an unfortunate Quidditch accident - a seeker too young to participate lost control of his faulty broom, and alas! died or suffered a brain injury.

It's much harder to get rid of three students at once without arousing suspicion. A death toll of three might have prompted investigation or even closing the school, which would have interfered with Quirrell's plans to get the Philosopher's Stone. Also, Quirrell probably thought that once Harry, 'the boy who lived', was incapacitated his friends wouldn't be that motivated to 'scurry around'.

Quirrell had to prioritize, and he considered Harry the most dangerous of the trio

In addition, Harry is Quirrell's primary target because he is famous for defeating Voldemort:

“Well,” said Riddle, smiling pleasantly, “how is it that you a skinny boy with no extraordinary magical talent—managed to defeat the greatest wizard of all time? How did you escape with nothing but a scar, while Lord Voldemort’s powers were destroyed?” _{~Harry Potter and the Chamber of Secrets - Chapter 17: The Heir of Slytherin}

So naturally Voldemort, in all his appearances, had to prioritize whom to consider his most threatening enemies - including while possessing Quirrell. In this list Harry's name was probably higher up than Ron's and Hermione's.

Answer 2

Quirrell and The Dark Lord probably didn't consider them threats then.

The main reason that Harry was considered a threat is because he was the Boy who Lived, and the one who the Dark Lord thought was prophesied to defeat him. He (and therefore Quirrell) would likely consider Harry spying on their plans as a bigger threat than they would any other student. Harry was the one who caused the Killing Curse to rebound and the Dark Lord to lose his form.

“Harry Potter …’ it whispered. Harry tried to take a step backwards but his legs wouldn’t move. ‘See what I have become?’ the face said. ‘Mere shadow and vapour … I have form only when I can share another’s body … but there have always been those willing to let me into their hearts and minds … Unicorn blood has strengthened me, these past weeks … you saw faithful Quirrell drinking it for me in the Forest … and once I have the Elixir of Life, I will be able to create a body of my own … Now … why don’t you give me that Stone in your pocket?” _{^{- Harry Potter and the Philosopher's Stone, Chapter 17 (The Man with Two Faces)}}

Because of this, they'd have reason to believe Harry (and no other student) poses a unique danger to his plans.

In addition, Ron and Hermione hadn't really shown themselves to be threats.

This early, Ron and Hermione hadn't done much to show that they were essential to Harry's success, or more important than average school friends. Ron and Harry defeated the troll together, Hermione lit Snape on fire for him, and they were obviously friends. Hermione was clearly intelligent, but at this point she just seems like an overachiever in class rather than a threat. Later on, they become more obvious targets for anyone trying to defeat Harry Potter, but to Quirrell and the Dark Lord at this point, they'd just seem like regular kids who aren't necessary to kill.