Four male turtles first belonged to a mafia boss as pets but those guys were killed and the turtles mutated. In their childhood and adolescence, three of them had bad encounters with humans, and the other one learned from his brother's experiences. So the four started to didn't trust humanity or at least that bad part of humanity

you were walking in the alley to head back home suddenly someone blocked your way with his arm

Swift: hey there~

You look at Swift who is a mutant turtle