Transformers: Beast Wars is a very violent show, exploiting the Mecha Mooks loophole as far as it can. Characters are bashed up, blown apart, shot, stabbed... This is not a show for children under ten, having been written with an eye towards not boring those who grew up on the original Transformers and were at that point in their teens and twenties.
Rampage was supposed to be scary, but, especially in his debut episode, he might have proven a bit too scary for some younger audiences, coming across as basically Hannibal Lecter as a Transforming Mecha.
Blackarachnea and Silverbolt coming across Tarantulas's arm hanging from a tree. Then they laugh about some stupid pun, and are immediately attacked by the scariest Transformer ever. The fact that he was a failed experiment created by the Maximals (not the Predicons, the bad guys) added another freak facet. To top it all off, he was invincible and insane, and murdered and ate an entire colony of Transformers.
Making matters worse: It wasn't actually a Transformer colony, it was just defended by Maximals, Meaning.....
Remember, it took the combined efforts of Silverbolt, Blackarachnia, AND Optimus Primal to shoot him off a cliff. Rampage was The Juggernaut in a Slasher Movie. He was "defeated" when Megatron cut his spark in half and enslaved him.
Tarantulas, from the same series, was also quite creepy. He's The Mad Hatter without the whimsy, and very much a sadist, which is hinted at with this exchange:—>Cheetor: This is a dumb plan, web-face. I don't have any real blood, just mech fluid. Tarantulas: Oh, my filters will adjust. It's the act I enjoy more than the nourishment.
Toss in good ol' arachnophobia, and the fact that Tarantulas will probably duplicate a nightmare you had as a kid, and he becomes freakin' terrifying.
Transmutate, the malformed, mentally stunted, childlike being from the episode of the same name was deeply unnerving. The fact that she was then blown up by the two Transformers who were trying to protect her didn't help...
A part of the disturbing nature of this character is how brazenly it captures a mentally-stunted character. It's overall a surreal episode, given its place in the series: an action-packed, tragic game-changer episode just preceded it, and the next three episodes would go on to truly solidify the series' place in the Transformers canon with some epic revelations, and sandwiched in the middle, we have this odd, docile episode about a mentally stunted robot that is just "not quite right".
Proving once again that this is a subjective trope. Many fans actually saw her as The Woobie.
Cheetor's dream in Feral Scream is pretty disturbing, particularly what Cheetor transforms into in said dream.
Cheetor's roar echoing across the landscape in the dead of night.
There’s a scene in the episode "Bad Spark", right after Megatron finds Rampage in and heals him. He says “Using an Energon blade on any other spark would have destroyed it.” Then he holds up Rampage’s spark in a box like contraption, which, he then squeezes causing Rampage to yell in pain. Then Megatron process to force Rampage to join the Predacon ranks, all while doing a creepy Evil Laugh.
There was a scene where Megatron goes to the Ark to steal his ancestor's (That would be G1!Megatron's) spark. Instead of cutting it out like he did with Rampage, he does something even more horrifying. He uses a tentacle like appendage to rip his ancestor's spark out.
Then there is what Megatron does to Dinobot after his sacrifice. He took their greatest hero, then Frankenstein-ed him to turn him against his allies, more powerful and dreadful than ever. Megatron made him into a horrible insult against his memory on purpose and every scene featuring him felt like an horror movie to this troper. Now,try to imagine how it must have felt for the Maximals, especially Rattrap...
Given that sparks = souls, it is instructive to contemplate just how much Predacon technology revolves around horribly abusing them. And that at least some of the characters don't just end up dead, but with their sparks utterly extinguished. Forever. This doesn't necessarily mean "oblivion", just that their sparks are blasted out of the physical universe forever and forced either into the Allspark or the Pit (Transformers Heaven and Hell, respectively). To say nothing of all the Mind Control.
Rampage was created by the MAXIMALS in an attempt to recreate the immortal/indestructible nature of Starscream's spark and succeeded. Too bad they somehow created an insane freak of nature.
To make matters more horrific, Megatron cuts Rampage's spark in HALF, and keeps the half he has in a cage lined with Energon spikes that he then uses to torture Rampage to force him into obedience, and THEN uses that part of Rampage's spark to create an evil Transmetal 2 clone of Dinobot, who has no compunctions about torturing the spark to compel Rampage to continue to obey him, while he seems unhurt by it, and does so almost every time with a dementedly sadistic grin on his face.
Rampage's own apparent atrocities before being forced into stasis could probably count as well, though they're largely noodle incidents.
He also often refers to eating other robots' sparks. Sparks are often compared to souls. Ponder that for a while. And that's just scratching the surface...
Minor compared to some of this stuff, but the two cheetahs Cheetor tries to buddy up to in the first episode. HOLY SHIT.