We're scared of robots that look almost, but not quite, human... because they remind us of walking corpses. And the deadliest killer robots are often the ones which exploit this zombie fear, before slaughtering us.
It's the "uncanny valley," the mysterious place where robots become lifelike enough to trip some of our systems for detecting fellow humans... but there's still something wrong with them. And the scariest killer bots often nestle malignantly smack in the middle of the uncanny valley, waiting to freak us the fuck out. Does this make them deadlier than other killer robots? Only if fear paralyzes you and makes it harder for you to strategize.
We're comfortable with robots as long as we can easily tell they're robots, and there's something abstract and mechanistic about them. But once a robot looks enough like us, but without all of our human mannerisms and foibles, we suddenly become uncomfortable. You can see it in David Byrne's singing robot, Julio. And in the guy who made a robot version of his girlfriend (left). (And she didn't dump him, why again?)