@blablablub
@Doomer
MC knows that SOMEONE in the town is powerful enough and corrupt enough to back these thugs. He DOESN'T know that it's (presumably) the town lord himself (yet), but he suspects that to be the case.
MC WANTS to know just HOW corrupt their backer is, so he left them alive.
Recall that his plan is to seemingly create a teleportation network that outright bypasses kingdom borders and whatnot.
MC needs to know whether the town lord is someone he could trust to cooperate with him.
For merchants, trust and reputation are everything.
No one's gonna want to come to a town where the lord sponsors serial rapists and murderers.
No semi-intelligent town lord (corrupt or not) is going to continue sponsoring rapists or murderers that claimed in public to be sponsored.
Whether the thugs are still around or not when MC comes back will determine MC's next course of action.