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?)
We've already charted the relationship between a robot's human likeness and how evil it is. But I'd argue that the scariest, and therefore most effective, killer robots are the ones which fall into that nether zone between artificial-looking and totally human. Just look at Masahiro Mori's famous chart. The "uncanny valley" refers to robots that remind us of corpses and zombies. So it's partly the stiffness and jerky motion, as well as the feeling of deadness, that creep us out.
Here are some examples of how robots that seem like dead humans, or humans lacking some "spark" of life, are scarier than other kinds:
For my money, the "skinjobs" in BSG were actually pretty scary and intense in the first couple of seasons, when they seemed the most inhuman. For one thing, in a sense, the "skinjobs" are dead, since we keep seeing them die and come back. The famous scene where Caprica Six is tossing Starbuck around is partly so awesome because Six's abnormal strength also makes her seem more jerky and suddenly less human. In later seasons, the Cylons started to seem more and more like regular humans, and even lost their ability to resurrect.
Just check out this scene with Ash, the creepy android whose secret corporate agenda is not terribly human-friendly. He even looks sort of like a zombie, with weird fluid coming out of his mouth:
Doctor Who, "Robots Of Death":
I'll be writing more about this 1977 storyline later on this week, but I would be remiss to leave it out here. The eponymous deadly robots in this story are deliberately designed to look cold and unresponsive, so we're not really surprised when they "suddenly" go all red-eyed and start strangling people. But the true genius of this story is that it verbalizes just why people are so terrified of lifelike robots: it's their deadness. The newly invented disease "robophobia" (or Grimwade's syndrome) takes this fear of corpse-like stiffness and pushes it to its farthest extreme.
Let's accept, for the moment, that the artificial "Replicants" are actually robots in some sense. (It's kind of vague in the movie, IIRC, and they definitely have a biological component.) They're stronger and smarter than regular humans, but they're also closer to death, because they have an in-built expiration date.
You can't get more zombie-like than the original T-800, played by Arnie in his prime. Arnie has never been stiffer, and deader-looking, than when he plays this unstoppable murder machine. Especially once he gets some face damage, and he starts wearing those sunglasses, so he has absolutely no expression whatsoever. You shoot him and blast him, and he keeps getting up... because he's already dead. Not to mention, he turns into a walking skeleton, which doesn't hurt, either.
OMG this is the clincher as far as I'm concerned. Yul Brynner is the walking dead in this movie. Look at his frozen expression and how it slowly turns into a rictus smile when he says "Draw." Now I'm going to have nightmares of him singing "Shall We Dance" while dismembering people:
Star Trek, "What Are Little Girls Made Of?"
This is is the classic zombie-bot strategy: have one robot who clearly resembles a dead guy, and give him a creepy name like Ruk, or Krob, or Glop, and then nobody will notice that the rest of you are all zombie robots as well. Just look at Dr. Korby's stiff, jerky motion and empty eyes. Even Shatner looks more deathlike after he's a robot:
There was a lot wrong with this Will Smith Converse All-Stars vehicle... but one thing it got right was the fact that its deadly robots look totally like ghosts, pale and almost translucent... and they have the nearly-but-not-quite human expressions as well:
Eve Of Destruction:
Those staring eyes. The way she flirts and laughs with absolutely no real expression. The way she covets people's fashion items. She is a hot dead lady, and she's out to kill you. Her only means of showing emotion is to make her eyes even bigger and buggier. It's up to Gregory Hines, that dancing fool, to put her back in her box. Which sounds a lot dirtier than it probably really is:
The Bionic Woman's Fembots:
Like Eve in Eve Of Destruction, the "fembots" in Bionic Woman are dead girls. They're blank-eyed, stiff and heartless, and there's something just "wrong" about them, even before their faces inevitably fall off: