Don't forget that teenagers often think they're invincible, and while Levi doesn't think that he is very sure in his abilities as a soldier and a fighter. (And nothing else.) He doesn't know about Vincent turning into other monsters, though, so he doesn't factor that in.

He rubs the back of his neck. "I guess I kind of figured you wouldn't let it get to the point where you're so hungry you're just jumping the first person you see. Unless its a Moon Warp, I guess, those are...weird."