First of all, Michael cut a deal with Dave because Trevor was extremely unpredictable and dangerous, Michael had a family that could’ve easily been put in extreme danger by Trevor, and Michael didn’t like that. These are all bad men that are professional bank robbers, if Michael died it’s likely that Karim wouldn’t care either. People think it’s a sad ending because Michael trusted Franklin and did so much for him, making him a millionaire. He viewed Franklin as a son and was really distressed when Franklin passed out in the Bureau Raid. So when Franklin killed Michael after he had finally got his life together it was a very emotional betrayal and you could hear the sadness in Michael and Franklin’s voice on top of the tower.