I love players do the good thing without my prompting. I DMed a campaign once where they fought a company the whole time, kinda based on "The Company" in Heroes, then a bad guy appeared who could appear as one of the players and was killing off company members. The company proposed a temporary truce so they could team up and take this person down who was ruining both their teams' lives, but the players said F that, we're taking this person down, and then we're taking you down. I had plans for the future of the campaign for both directions, but I was sooooo proud they went that route. It was such a badass moment.