I was unaware that they "hated" each other. Then again, is there anyone that Metallica doesn't have a problem with?

Anyway, Mustaine was an original member of Metallica in the early 80's. Sometime between their bar gigging days and the recording of their first album, the band felt that Dave let alcohol take over his life. They woke him one morning out on the road, gave him his plane ticket and told him to get out.

If there's any animosity between the two groups, I'm sure that plays a large part.