Motive is what gives moral to a character’s acts. What a character does, no matter how awful or how good, is never morally absolute: What seemed to be murder may turn out to have been self-defense, madness, or illusion; what seemed to be a kiss may turn out to have been betrayal, deception, or irony.
We never fully understand other people’s motives in real life. In fiction, however, we can help our readers understand our characters’ motives with clarity, sometimes even certainty. This is one of the reasons why people buy fiction — to come to some understanding of why people act the way they do.
A character is what he does, yes — but even more, a character is what he means to do.