Her brothers killed him because he had sex with her BEFORE asking to marry her (and he was a Canaanite). There's no hint that they did it because she was not willing, but because he went about marrying her the wrong way, and he was an outsider.
If it was rape, I find it odd that Genesis 34:3 adds "3 And his soul clave unto Dinah the daughter of Jacob, and he loved the damsel, and spake kindly unto the damsel."
There’s a reason the story is ubiquitously known as The Rape of Dinah’.