Look at the interviews on EddieTrunk.com with Mustaine. He's not allowed to talk about the details of the lawsuit with Ellefson and how he settled the suit with him for a significant sum. That's the backup. Do the legwork yourself because it's all over the place. Additionally, Nick was led to believe that this was a reunion, thought he'd be part of Megadeth again, but Dave didn't have the same thoughts, so he was only used for the album and then released right afterward. It's called parsimony. After you look up the word, then go and do some legwork and research into how Mustaine and Megadeth pretty much fell apart. It's all over there.
It simply looks like you're such a Dave Mustaine homer that you'll deny anything that's happened that implicates him for any wrong doing, while he's nearly universally unliked by peers in the industry. Go to the Slayer FAQ and see why they don't like him.