I dont know what kind of massacre was in the Junktown, but i imagine situation, where one or more gangs attacked this guarded city.
My idea is, why not make something like "guarded city player militia". I know, that non stop guarding is not fun, but why not this?:
* if you have good karma in that city, you can be hired as a mercenary guard
* if city is attacked (by more than one, two, 3,???) players in special interval, you will be noticed by ingame message, that the city is in trouble and needs your help
* if you arrive, all attackers, their gang teamates (even if they are not attacking) and all players who arrived to city in the same group, will be marked by red colour (or something like this) and you are able to attack them without penalty that city guards will attack you too
* if you kill one or more attacker, you will get reward from some NPC, after the battle ends
* if you kill all of the enemies, the loot could be part of the reward for player guards
* no Karma reduction because of defending the city is neccesary measure