I never said Cold Morn should have done this or that, just presented the plausible motivations of Harmke and the rest that participated. They were scorned, resentful people looking for payback, you could even say justice, in a misguided primal way.
I started to write something on the killing in this kind of games in my previous post, but decided not to post it, so here it is. Yes we kill, and we kill a lot to finish quests and achieve the PC's goals, but during other quests we kill people because they try to hinder us, to stop them from doing we find deplorable, because they are an obstacle, or simply for personal gain. But in this quest we are asked to murder a man to satisfy the need for vengeance of an almost complete stranger. Now vengeance is a powerful motivator, and helping a friend achieve it is fine by me, but with the Devil of Caroc the request for assistance is almost the first thing she asks. They should have paced things better, like with Aloth first you witness a few of his outburst, get on friendly terms with him, then receive the quest, killing because a stranger asks you to for no return is not something people do.
And, if you are murderhoboing your way through the game, doesn't mean everyone else does to.