Maybe the only sexy part of violence itself is the adrenaline that pumps when a person feels fear. Gotta do something with that adrenaline.
Aside from that, I suppose a person who easily defends themself and those around them from an attack with confidence and physical finesse would be a turn on. Like a martial artist.
I've worked in a bar where they employed some really top notch bouncers. The good, professional ones (not the thugs) always made the work easier. Hanging out with these dudes after work and riding home on the train was cool. You thought, "if some punk comes in here, he has to get through these guys first." I can understand how that confidence and defensive ability would be a turn on.