Harry couldn't have killed Quirrell as he was unconscious at the time.
Page 214, Australian edition:
QUOTE
Harry jumped to his feet, caught Quirrell by the arm and hung on as tight as he could. Quirrell screamed and tried to throw Harry off - the pain in Harry's head was building - he couldn't see - he could only hear Quirrell's terrible shrieks and Voldemort's yells of, "KILL HIM! KILL HIM!" and other voices, maybe in Harry's own head, crying, "Harry! Harry!"
He felt Quirrell's arm wrenched from his grasp, knew all was lost and fell into blackness, down...down...down...
On the next page, in the hospital wing, Dumbledore tells Harry,
QUOTE
"I arrived just in time to pull Quirrell off you -"
So, Harry didn't kill Quirrell at all, self defence or otherwise. Harry's soul is intact!