Say what you will about Megadeth's Dave Mustaine, but he sure can hold a grudge. Four decades after he was fired from Metallica, he still holds a grudge. The amount of energy he has spent voicing his negative opinions about the band is impressive, though boring.
Mustaine simply needs to let that hate go. He has found a way to be highly successful with his own band, Megadeth. Have they reached the levels of success that Metallica has? No, and never will, and that bothers Mustaine.
On a recent episode of the Shawn Ryan Show, Mustaine went on at length about how hurt he still his by being fired from his former band. He would never use the word "hurt," of course; he has too much pride for that. But it is what drives him.
Dave Mustaine simply cannot quit his hate of Metallica
Mustaine made the claim again that he wrote most of the music that "made" Metallica. That opinion is not only misleading, but it is false. Metallica has had a sustained run of success since 1983, almost all of that without Mustaine. Metallica clearly made their own bones, and Mustaine appears to be simply jealous.
Still, why? He has been successful on his own, and maybe things would turn out differently for him and his former band if Mustaine stays a part.
Mustaine also implied that Metallica, whom he obviously sees as being incapable of writing their own songs successfully, stole "Enter Sandman" from another band, Excel.
The guitarist said, "(W)e (Megadeth) may not be as big as they (Metallica) are. Hell, their biggest song, 'Enter Sandman', go look up the band Excel right now. Look up their song — I think it's (called) something (like) 'Into The Unknown'. Pretty similar."
Therein lies the real issue. While in the same interview, Mustaine said he didn't care that he had been fired (wrongly, in his opinion) from Metallica, he obviously does. He has been ranting about them for decades, and at some point long ago, it just became rather sad and boring.
Metallica rarely responds to Mustaine's yapping because there is no need to. They don't need the Megadeth guitarist, but he still obviously needs them in some weird way. Maybe he should just keep playing music. People like that, but they don't like his constant gripes about a band he left in 1983.