I guess most GMs have had a group of murderhobos in their sessions before and there are quite a few nice answers dealing with how to react to them, these threads have nice answers, but I fear my problem is a bit too complicated for general punishment of the whole group.
I have dealt with quite a few murderhobos already, but mostly they were only a few in a group that strongly opposed them. A few months ago I started a new homebrewed campaign loosely based on DSA (in English: The Black Eye) rules, which started quite fine. Most of my players are rather reserved, so most actions of the group as a whole are decided by a few, who take up the initiative. One of these few is my star player. He has played Pen&Paper for years and it is a pleasure to have him in my party, as most of the more inexperienced players still lack the confidence to interact with the world vividly.
Unfortunately he has also quite a temper as well as a borderline aggressive vibe. He gets angry fast, takes things personal and gets grumpy and loud as soon as things don't go his way. NPCs that are hostile or offensive to him end up on his kill-list pretty soon. This happens quite regularly, as the group plays as Witchers, which aren't treated with much hospitality. He doesn't care about NPCs, plots to kill guards and royalty and beats shopkeepers into submission, because he claims they disrespected him with their prices. He played a murderous viking in a one-shot adventure I held before and now has a rather feeble and laid back character, but doesn't play him much different.
- Due to some unfortunate accidents with black magic the group ended up wreaking havoc to a local inn, which caused the other innkeepers in my village to stop serving them. My problem-player saw this as disrespectful, knocked that guy out cold, prohibited (threatened) the rest of the group from tending to him and 'took over' the inn.
- Due to some critical fails in bartering, my problem-player convinced a local scribe – twice – that his map of the region is worth more than he is currently demanding, after raising the price to something my group couldn't afford. The problem player tortured that poor scribe until he fainted and would've killed him, if not for another player.
- Having anyone look down upon him will make my problem player immediately plot to kill him. I created an easy-to-antagonize Baron for whom they are currently doing some jobs. Each time they meet him I have to gently remind my murderhobo about the castle full of well-armed guards, who definitively can end their adventure on the spot, as soon as they notice him attempting an attack against that baron again.
I thought about several ways to get him to stop treating these people poorly, but I doubt any of them will help. I thought about:
Reminding him to act according to the rather gentle nature of his current character (which he made himself)
Problem: His personality clearly somewhat reflects in his character and I doubt that this will be a long term solution. I already had a talk with him, after he complained - in character - how the dungeon boss deals too much damage and then left the fight, slamming the door behind like an offended teenager. It got better, but not for long.
Removing him from the group
Problem: As I said he is one of my most active players. He advances my plot and the group would greatly suffer without him. (We had sessions without him, so I know the others aren't just intimidated)
Punish them for their crimes
Problem: I plan to do that somehow, but I fear that general punishment will have quite the opposite effect, as my violent player would get even more mad while the others are dragged along into committing even more crimes. I even had one campaign come to an end by mass-suicide, because I had a few murderhobos in my last group, who thought the best idea to break free from a encirclement of a superior enemy was to behead, gut or ignite the few important prisoners they had taken previously.
Only punish my problem-player.
Problem: I didn't find a way to not make it look like I am picking on him, which would escalate the situation. It also wouldn't make much sense for the villagers to not condemn the whole group of strange outsiders, if their supposed leader has just reinforced their prejudices.
The group doesn't necessary agree with the actions of my murderhobo, but doesn't oppose them either, so they just follow.
Since playing as Witcher was my idea, as I liked the whole 'Monster Hunter' setup, I asked my group on several occasions if they would like to abolish the Witcher-plot and play on as brigands or mercenaries, since they didn't seem to internalize the whole concept, as none of them ever read or played the source material, but they decided they would like to stay as Witcher.
I don't mind them stirring up some trouble. In fact Geralt of Rivia himself has more than enough brawls and fights with harassing peasants in the games and books. But maiming everyone who makes abusive remarks to the Witchers makes the whole campaign stressful and boring to me, as no matter what encounters I plan, they'll end up brawling with the city guard. And yet again, none of the other players seems to be in for that kind of progression.
I think the best way my problem-player would reflect on his previous behavior to be less violent, would be if he met opposition from within the group. Although nobody seems too happy about going in the direction my murderhobo is leading them, nobody stands up against him either.
How can I help my rather reserved players to discipline their murderhobo, if they don't agree with his violent ways?