The trope: link

In the episode "Dirk Savage, He hired the self-titled mutant hunter to capture mutants claiming them to be a menace to society with no regard to them being good or evil.
However, not only is Howard a mutant himself, he planned to build an army of brainwashed mutants in hopes of mutating and controlling the entire human race, this leads to Don and Raph stopping a vengeful Rahzar from killing Dirk in order to convince him to turn against Howard.