I don’t see the reason in the actual Scripture though. Nevertheless, they didn’t kill him because he violated their laws about rape. They killed him because he had sex with their sister before marriage, and even if she’d been willing, the result would have been the same. She was useless to them once she was no longer a virgin. Damaged property. That’s how women were seen.
The original Hebrew is pretty clear, even using understood euphemism. It’s been rape for 3500 years, until now.