Ok, my ideas may sound a bit morbid, but i think they're also a good fit anyway... you could show a knife or something with blood on it and maybe have the title in the blade? Or an empty room with blood dripping from the ceiling? Or if the assasin kills a girl or something, you could show a shattered mirror? You could also do something with a gun, maybe a cool picture of a bullet flying really fast? There's also poison that you could show, like, a spilled bottle or something. Hope this helps! I really like your idea for the story!