Because it’s implied that Quirrell thinks Harry used it, it follows that Quirrell thinks it was feasible for Harry to use it. IIRC it’s mentioned elsewhere that the killing curse is powered by killing intent and not magical power. Harry may not have wanted to use the killing curse either because he didn’t have enough confidence in his killing intent (he has to want to kill something terminally, not instrumentally) or (and this is stretching) because he didn’t want to deal with the political repercussions of being known for using the killing curse on top of everything else.
It’s also possible that to Harry, ‘tricky transfiguration weapon’ is more mentally available than ‘THE KILLING CURSE’. Also, for the same reason, Harry might not have learned it.
Quirrel knows the killing curse better than Harry does. He also is likely to have information about Harry, which Harry lacks.
So the fact that he is sure Harry could pull off the killing curse does not mean that Harry has any reason whatsoever to share his confidence. And having it fizzle would be fatal.
“Finite the rock” on the other hand is going to work. So he went with that.
Because it’s implied that Quirrell thinks Harry used it, it follows that Quirrell thinks it was feasible for Harry to use it. IIRC it’s mentioned elsewhere that the killing curse is powered by killing intent and not magical power. Harry may not have wanted to use the killing curse either because he didn’t have enough confidence in his killing intent (he has to want to kill something terminally, not instrumentally) or (and this is stretching) because he didn’t want to deal with the political repercussions of being known for using the killing curse on top of everything else.
It’s also possible that to Harry, ‘tricky transfiguration weapon’ is more mentally available than ‘THE KILLING CURSE’. Also, for the same reason, Harry might not have learned it.
Quirrel knows the killing curse better than Harry does. He also is likely to have information about Harry, which Harry lacks. So the fact that he is sure Harry could pull off the killing curse does not mean that Harry has any reason whatsoever to share his confidence. And having it fizzle would be fatal. “Finite the rock” on the other hand is going to work. So he went with that.