cartoonishly evil

An act that is so immoral, unethical, mean-spirited, spiteful, petty, low blow, etc. that it seems more comparable to the classically over-the-top schemes of a cheesy villain in a children's cartoon than the actions of a real live person.

"Did you hear about those activists who beat up a homeless man and stole his puppy?"
"Wow, that is cartoonishly evil."