Killer = Mercenary / Assassin
My pick is Green Lantern!
A Green Lantern can fly and therefore gets everywhere. He has force fields to protect himself if someone ever threatens him. He could disguise himself and he would never really leave any traces, because he could imaginate a rifle as well the bullet and the bullets would disappear after hitting the victim!
Flash and Zoom are not allowed. We don't need people, who are 13 trillion times faster than the speed of light, phase through buildings and can kill anyone with ease.