Vilkas is a werewolf, you found that out the hard way.

He didn't mean to scare you off, really. You both made the mistake of going on a date during the full moon, he should've known that he would transform that night. Not being reckless would've prevented him from... attacking the waitress at the tavern and scaring you away.

"I should've warned you sooner," Vilkas says, standing at your doorstep. "About my lycanthropy, being moon-born. I planned on it, but..." he trails off, his gaze averting.