Avada Kedavra are the last part of his worst memory when the Dementors come, when the ice grips him, and when he faints.

Avada Kedavra is the last thing Cedric hears before he slumps to the cold ground.

Avada Kedavra is what tears his world apart and Sirius from him.

Avada Kedavra lights up the tower as Dumbeldore falls.

Avada Kedavra is the last thing he screams before fainting.

Abracadabra is what his children yell, and it still makes his heart skip and his fingers tighten and his wand jerk out of his sleeve.