Why does Lennie kill Curley's wife?

Curley's wife observes that Lennie is "just like a big baby” and invites him to stroke her soft hair. Lennie begins to feel her hair but presses too hard. Curley's wife begins to panic and struggle: this also sends Lennie into a panic. Lennie holds her to silence her screams. Lennie fears getting in trouble so he shakes her violently inadvertently breaking her neck.