He's bitter cause his big ego got bruised during an extremely formative time where he had formed his identity around a certain thing that went bad. It's like your first love but for your identity and career instead of romance. Thing is Metallica isn't big enough for a Hetfield and a Mustaine. That's too many riffmonsters, he was built to compete against him not stand by his side.
>>126956953S tier:
Kill 'em All, Master of Puppets, Peace Sells, Rust in Peace
A tier: Ride the Lightning, And Justice for All, Black Album, Killing is My Business, Youthanasia
B tier: SFSGSW, Countdown to Extinction