Cameron's stoic emotionlessness while doing horrible things just goes to show how terrifying Terminators really are.
Not to mention when she suddenly starts emoting, like this scene in the Season 2 premiere. "John! Please, please don't! I love you! Please-"
And a certain liquid-metal terminator with a penchant for killing men with their pants unzipped.
And the terrifying birth scene of a certain newly-repaired terminator emerging, skinless but not fleshless, from a literal bloodbath, and cutting open his eyelids to reveal his glowing cybernetic eyes.
You're never safe.
"Hello, Mister Ellison. My name is John Henry."
"Your hair... it's so pretty...." And you never thought it was possible to make Les Yayterrifying.
Speaking of terrifying Les Yay, the relationship Jesse has with Riley is essentially a case of domestic violence. Jesse constantly forces Riley to do her bidding and hits her when she disobeys. Jesse also follows a common abuser pattern where she will occasionally show affection to Riley in between her more abusive behavior. There's also the fact that Riley is underaged and Jesse is in her thirties and is clearly taking advantage of Riley. Jesse always reminds Riley that she "owes" her for bringing her from the post-Judgement Day future. Riley also seems to have some manner of affection for Jesse and at one point brings up the idea of them getting an apartment and "being together". Whether Riley has romantic feelings for Jesse is open to interpretation, but either way it's clear that Jesse is taking advantage of her. Later on it's revealed that Jesse was trying to get Riley to provoke Cameron into killing her so that John wouldn't trust Cameron anymore. Jesse does actually kill Riley and that just reinforces the domestic violence themes.
Well during that fight scene between Jesse and Riley, Riley yells and Jesse saying that she trusted and loved her. It is open to interpretation if Riley loved Jesse in a romantic way, but either way Jesse was definitely taking advantage of Riley's feelings for her.
"What... is...all this... about?"
One little puddle of pee when Savannah's "mother" simply speaks calmly to her.
Cameron abandoning the Russian siblings to be killed by gangsters in "The Demon Hand".
Weaver killing all those people in the "air-conditioning factory".
Allison slowly reverting back to Cameron in "Allison from Palmdale".
* After Allision's second escape attempt the camera pans up to show Cameron's face. The look of shocked horror Allison is sporting must definitely have been mirrored on more than one viewer's face.
Cameron has this really, really creepy line late in the first season after she kills the assassin that Sarkissian sent to kidnap herself and John.
"He said very little, and then he was quiet."
Cromartie, early in the first series, regrows his flesh in a literal bloodbathand emerges from the blood, flesh regrown, as a lumpy, distorted nightmare. Worse still, the poor scientist watching all this has to use a scalpel to cut his eyelids apart, revealing the machine's glowing red eyes underneath. And then there's a disturbing mention in the next episode of where Cromartie got his new human eyes from...
The bit where Cromartie kills the guy whose face he has stolen by appearing behind him and smashing his face in the mirror. Then he mimics the poor bastard's horrified expression as he died....
The Season 2 premiere, where we learn that damage from the car bomb is causing Cameron to try to kill John.
Cam executing the 3 burglars in "The Brothers of Nablus".
In-universe; Sarah's reaction to learning about 9/11 - remember, when she, John, and Cameron made their time jump at the end of the first episode, it was from the late 1990s to what was then present day. The notion of killer robots doesn't faze her, but the idea that humans could willingly conceive, and pull off, something like what happened that day horrifies her. In her voice-over, she says that if she had been around to see the Towers fall, she would have thought it was Judgment Day right then and there.