Greedly, Plunder and Sludge we're all in it for the money and didn't give a shit about anything else.
Nukem was mutated into a nuclear monster that derived power from radiation and was pretty much driven insane by his transformation.
Skumm was a rat monster who hated humans and enjoyed fucking with them, usually by turning them against each other. His end goal was taking out humanity and their civilization and ruling over what was left with his rat man slave race (which were occasionally implied to be forcibly transformed people).
Blight was a mad scientist, occasionally in it for money, but for the most part she just enjoyed wrecking shit with her inventions like an overgrown child (best exemplified in the virtual reality episode or the one where she steals Gaia's powers).
Zarm was an Earth spirit who was corrupted into a kind of social Darwinist war God who had it out for Gaia because she was his antithesis (and I think she banished him from Earth or something in the past, I forget).
The Slaughters were just asshole poachers, basically an entire family of Kravens.
Captain Pollution was literally Bizarro Captain Planet.>>95944788
Sludge did end up going legit in the end, I think after FINALLY being convinced that he could make more money that way.