After seeing Metallicas some kind of monster documentary it has to be said that Mustaine did not come off well. He appeared as a bitter man who has never been able to get over or forgive his ex-bandmates for throwing him out of what became the worlds biggest band, and consequently not enjoying any of the massive success he achieved with Megadeth.
His constant battles with drugs meant and controlling personality meant he has alienated, well pretty much everyone he has ever played with.

But in saying that he has made some of the best heavy metal albums off all time, and all the while has had to deal with living in the shadow of Metallica, a multi million selling, twenty year inferiority complex!
Always controversial, never boring (even if the last couple of albums have been a bit skew whiff) and a pretty nice guy when hes not angry.

I'm sorry, I was going to answer your question, perhaps with something inciteful and though-provoking, but upon reading this over...

Did you say that Metallica was the world's biggest band?

...I think not.

Mustane really threw himself away. Megadeath is a joke to almost everyone. But then again, Metallica is too now. I didn't see the documentary, but I did hear a lot about it. All of it stunning but none of it surprising. I think its kind of funny to get a bunch of people who used to feel like they were on top of the world and have them all sit down talking about whos fault it is that they aren't anymore.

I haven't seen the documentary, but from what I've read and seen over the years, I would say "just plain tool"