Early into my first campaign with my current group I had a similar situation. We were in our first dungeon and our group was arguing about what to do next because we’d just lost an NPC due to our stupidity (our dm actually murdered him because he didn’t like him but it was played off too well) and my character discovered the secret during their argument. So I went off on my own (usually impossible because I’m a giant mushroom but they were deep into their argument) and found the boss who just took out the NPC. I thought the description of the boss was too awesome to kill so I tried befriending him. And succeeded. He’s still an NPC in our current campaign and if he ever dies I’ll kill the whole group then myself.