I think it is honestly Kraang Prime as while Overmind and Kavaxas explicitly and was implied respectively to have a higher attempted body count, Overmind's plans were just a universe, Kraang Prime wanted to rule the multiverse, brainwashing his entire race to be mindless copies of himself and then mutate all the other beings using the stabilized mutagen which kills the victim's original personality, why, because Prime brainwashed them, when Kraang mutated himself (to which he gained the psychic powers and size), his original personality isn't like dead, he just went mad with the power he has now. This is why despite Overmind having a higher attempted body count, Kraang Prime is arguably worse. And Kavaxas' implied method to conquer by having his demons destroy everything and rule the remains is as told, implied, because this is his method of conquering Earth and he swore to rule the 10 dimensions. Seems worse, right, as I told, it is implied, Prime's methods are explicit.

Ofc, he will be more heinous than the Rat King. I mean, feel sad for like being too mean since he is what is to be the show's first truly heinous villain and my favourite PE from the show, then again, his resources are very low and what he did to Rockwell and Splinter is just awful, that is excluding what he planned to do with his own race. The Rat King is easily the least heinous out of this group (not saying much), but still imo, he is a stronger keeper than Kavaxas, I mean, if he didn't drain the souls of Tatsu and the Hammer, toss some of Don Viszioso's henchmen to their deaths considering their lack of plot armor and that they aren't ninjas or just strong, burning some of them to death, or that if his end of the world wasn't the reason Tiger Claw made a truce and Shredder literally sacrifices himself to get revenge on him for brainwashing him (being the only villain the guy loathes and is disgusted by, ikr, surprising), he wouldn't have counted and that if there ever was a removal on him, I will say, "don't agree but understandable" compared to if there was one for any of the other villains.

Clarification of the title, I forgot to add 2012, so this is incase someone is disappointed Utrom Shredder isn't here.

Now I am gonna be in the lesser part for now, but in all honesty, Overmind really.

But on one half, as mentioned in your recent NPE proposal, Kavaxas did manage to piss everyone off- XD

Overmind IMO.