Additionally, Quirrel states in Roles, Pt 1, that Harry has insufficient raw power for the Killing Curse.
The Defense Professor inclined his head. “I shall teach you almost any magic you wish to know, Mr. Potter. I do have some limits, but you may always ask. But what specifically do you seek? You lack the raw power for the Killing Curse and most other spells deemed forbidden—”
Additionally, Quirrel states in Roles, Pt 1, that Harry has insufficient raw power for the Killing Curse.