Why was Quirrell scared away by Firenze?

Question

‘AAAAAAAAAAARGH!’

Malfoy let out a terrible scream and bolted – so did Fang. The hooded figure raised its head and looked right at Harry – unicorn blood was dribbling down its front. It got to its feet and came swiftly towards him – he couldn’t move for fear.

Then a pain pierced his head like he’d never felt before, it was as though his scar was on fire – half-blinded, he staggered backwards. He heard hooves behind him, galloping, and something jumped clean over him, charging at the figure.

The pain in Harry’s head was so bad he fell to his knees. It took a minute or two to pass. When he looked up, the figure had gone. A centaur was standing over him.

_{Philosopher's Stone - page 187 - Bloomsbury - chapter 15, The Forbidden Forest}

At the time Harry encountered Voldemort in the Forbidden Forest, as Quirrell drank the unicorn's blood, Quirrell was probably at, or close to, his magically strongest, and he also the benefit of Voldemort's powers (although they are weakened).

Centaurs, on the other hand, don't seem to have overt magical abilities -- they read the skies and rely on the bow and arrow as weapons.

So how is it that Firenze, without magic, was able to drive away Quirrell/Voldemort? It would seem that Quirrell/Voldemort could have easily incapacitated Firenze with magic and gone after Harry.

Why was Quirrell scared away by Firenze?

Answer 1

Quirrell wasn't scared away by Firenze alone.

• Hagrid: He was close by. It would have taken a lot to take him down.

• Centaurs: The woods are full of them.

• Firenze: Probably has ways of protecting himself and maybe is more resistant to spells than a human. Also, he might be faster.

Seeing Firenze woke him up to the reality that he wasn't alone with Harry and strong people were around. If he fought Firenze then others would come and he might be caught, cover blown and without a body. There were well trained wizards in Hogwarts if he fought one then more would come.

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Also, Voldemort was weakened (that is why he was drinking unicorn blood). And he might have not been ready to face Harry yet, that is why he was waiting until he got the Sorcerer's Stone so that he was stronger and had a physical body.

He had a borrowed body. When he used to borrow snake bodies they were too weak and would eventually die. Imagine what the extra strain of 2 of extra bodies was doing to Quirrell. He realized he was weak.

Also, Voldemort probably wasn't fully ready. He wanted to kill Harry, he hadn't been fully prepared. He wanted to be prepared that time. Quirrell might have wanted to attack and gain favor with Voldemort, but was called off. If Harry died people might suspect Voldemort and the community would be on full alert.

Answer 2

This is very simple. Voldemort was using Quirrell at this point in time to get the philosopher stone. Killing Harry, Draco, and a few centaurs is going to put Dumbledore on high alert, therefore complicating, if not foiling his chances of the philosopher stone. As well as potentially alerting Dumbledore specifically to the presences of Voldemort. We also see in the books that Centaurs are not afraid of magic, and if multiple centaurs are firing arrows at you only one needs to hit. So again simple tactics you don't fight archers in a Forest where they can be behind any tree, and they can clearly see you.