Also I'm not sure I understand why you need them to have low responsibility. I think only classes with the Guard checkbox set will attempt to arrest, regardless of responsibility.
They need low responsibility so that they don't pay attention to the lawful status of targets when picking who to fight. Using a low responsibility, rather than just flagging one of their factions as evil, will still make it a crime if they are attacked while still allowing them to help out another friendly NPC even when doing so would make them a criminal.
Just to make a quick list;
All the militia are part of their own unique faction who has a higher than 50-100 disposition towards itself and probably something like 30 toward townspeople faction?
All militia are setup with 10 aggression, 100 confidence, 60 energy, 20 responsibility?
All of the townspeople are part of their own unique faction, have no disposition boost toward themselves and a -10 disposition toward militia?
All townspeople are setup with 5 aggression, 40 confidence, 40 energy, 20 responsibility?
Neither the townspeople nor the militia factions are setup as evil?
In this setup, if the player attacks a militia member, all of the militia should attack the player and all the townspeople should run for cover.If the player attacks a townsperson, the one being attacked should either fight back or try to run away with none of the other townspeople helping, but if someone in the militia sees this, they will start attacking the player. If the militia attacks a townsperson, all the townspeople will run in terror, and other militia will join in. All the while, no bounty or crime gold will be given to the player or any NPCs, but it would still count as assault and/or murder.