Baxter Stockman is the primary antagonist of Teenage Mutant Ninja Turtles (2014 3DS game). He supplied the Rat King with Mutagen, created Slash, and developed the hallucination gas that forced the Turtles to fight an illusion version of Shredder whilst he sprang the real one from prison.
In various note pages around the game's extended missions, it is revealed that Stockman had once been a noble scientist, full of optimism and hopeful to help the world. However, his experiments began to skirt the line of morality, and eventually he fell over the edge.