This is the reason why he was fired:
" Mustaine's membership in Metallica would last less than two years. Brian Slagel, an early manager of the band, recalls in an interview: "Dave was an incredibly talented guy, but he also had an incredibly large problem with alcohol and drugs. He'd get wasted and become a real crazy person, a raging maniac, and the other guys just couldn't deal with that after a while. I mean, they all drank of course, but Dave drank more...much more. I could see they were beginning to get fed up of seeing Dave drunk out of his mind all the time."
One of the most reflective moments of Mustaine's time in Metallica was when he brought his dog to rehearsal. The dog jumped up onto the car of Metallica bassist Ron McGovney and scratched the paint. Hetfield allegedly proceeded to yell at Mustaine's dog and kicked him\her in anger. Mustaine responded by physically attacking Hetfield and McGovney and yelled at Ulrich.Mustaine was fired following the altercation. The next day, Mustaine asked to be allowed back in the band and was granted his request. Another incident occurred when Mustaine, who had been drinking, poured a full can of beer down the neck and into the pick-ups of Ron McGovney's bass. When McGovney tried playing it, he received an electrical shock which he claims 'blew him across the room and shocked the hell out of him'. McGovney then told Mustaine to leave his house, and quit the band."