In “Teenage Mutant Ninja Turtles: Mutant Mayhem,” Splinter is portrayed as less of a wise, old martial arts master and more like a single dad raising four kid, a guy who is just perpetually anxious about the outside world harming his boys. And he has a good reason to think that.
During a flashback scene, we learn that this version of Splinter is literally just a regular rat whose best friend was a cockroach that got stepped on by a human (and then Splinter ate his friend’s remains, because he was still just a regular rat). He found some baby turtles covered in ooze in the sewer, and because they’re the only things he’s met that didn’t immediately hate him, he decided to raise them as his own. Now mutated himself after touching the ooze on the turtles, Splinter eventually introduces the boys to the human world. But the humans screamed in fear and disgust and threw garbage at them. This makes Splinter decide humans are true monsters, not to be trusted.