Ever notice how players don’t trust anyone and just kill everyone over anything and with any excuse? Well, it might be your fault.
Now, I would like to note that this might not be only you. If players have learned from through previous games and GMs that violence is often the answer, perhaps the only answer (I’ve played under GMs, who have actually forced combat encounters, when players were actively trying to find other solutions), these players will have learned certain modes of operation, and unlearning them might be difficult.
Obviously certain games encourage this with rules that emphasize combat. If that’s what your character is set up to do starting with character creation, that’s what you want to do. Therefore, if you are using a system which has combat in it’s core, you have to expect player characters to want to fight.
This isn’t the only reason. RPGs don’t often have very many casual encounters. This is often fine. If you look at stories, people only meet for a reason. Just going through the everyday life of a character is generally boring. For that reason you push the interesting moments to the fore.
Now, characters will have opportunities for other kinds of interactions. However, if they are generally either boring, hostile or (way too often) untrustworthy, the players will act accordingly. This is a difficult balance. You have to make interesting characters, but they can’t steal the scenes. The spotlight should still be on the player characters.
However, in order for the players to be interested in something other than simple goals of gathering loot and experience, the world needs to be rich, or at least have the potential for that. You should create opportunities for the characters to meet people, who are not going to backstab them at the first chance.
But those encounters need to be interesting. They need to stand out between the bloodshed. Or, if you try to avoid the bloodshed, they need to be interesting enough on their own.
One way to do this is actually making bloodshed be more part of the whole than separate. What often happens is that when the combat starts, the mindset of the players changes. Maybe you bring out maps and miniatures, or change the music, but the message is clear. You are not in the story anymore, this is something different.
This should never happen. Even in the best action movies the action is not separate from the rest of the movie. The story is just told differently. You should work to divert the expectations here. If a player decides to rough someone up, don’t let them break the immersion. Just let them rough up someone, if that’s what you think would happen in the situation.
You just shouldn’t let that become the norm or you should at least encourage different kinds of approaches. There should be actual consequences to behavior you want to mitigate. If the player wants combat and is rewarded for his behavior with more combat, they don’t actually have a reason to do anything else. So, if your character gets a reputation for lack of respect for others, just make sure they know they’ve lost certain lucrative opportunities because of that or something like that.
But I do think the key here is that they need positive interactions. Not every NPC has to be an asshole to the characters, as many unimaginative GMs like to do, nor do they have to be there just to wait for an opportunity to stab the PCs in the back, literally or metaphorically. This will hopefully teach them to trust others instead of only thinking about what they can gain from killing each person they meet.