Kaynar and Addicus. Thinking back to Jackalman and Monkian (respectively) in the 80's version, they were bumbling fools and not the least bit threatening. On the other hand Kaynar and Addicus are physically imposing and competent fighters, but as if that was not enough, Kaynar is a giggling, sadistic, psychotic serial killer, and Addicus is not only a killer himself, he also enjoys preying on other sentients.
Addicus's introduction should get special mention. He's seen hanging by a rope from a cliff as a tribe of birds have sentenced him for death, for unknown (though pretty easily guessed) reasons. He is offered a last meal, but when he says, essentially, that he wants to eat the birds, it's skipped, the rope is cut, and he's sent plummeting to his doom...until Slithe's flying ship hovers up to the cliff, Addicus standing unrestrained on board. Addicus says "I believe I am owed a last meal..."...at which point we get a far shot of the bird village and are treated to the sound of many birds screaming in agony.
Lion-O's surprisingly graphic death by drowning in "The Trials Of Lion-O."
The Tiger Clan being killed and cursed with undeath by the Ancient Spirits of Evil. Lion-O and Tygra spent most of the episode in a literal ghost town.
The faces that the man with the "nervous disposition" makes in "Recipe for Disaster" if only because they come out of nowhere.
In "Soul Sever", Tygra gets his soul torn out and the result is a soul devouring robotic horror. Foreshadowed by Tygra who comments that the scenario was "...The stuff of nightmares."
The Soul Sever himself is a terrifying cyborg who's obsessed with putting the displaced souls of his family into machine bodies, and killed the Necromechers who made him a cyborg in the first place.
