Kerry King and Dave Mustaine have a long-storied relationship. King was a member of Megadeth for a cup of coffee in 1984 and ever since then, the two have had an interesting combo of friendship and rivalry.
In a recent interview promoting Slayer's recently kicked-off tour with Anthrax, King was asked by the Miami New Times if he ever saw Anthrax as a rival. He used the question to offer a slight dig at Dave Mustaine:
Yeah, for sure. When the "big four" came out [Anthrax, Megadeth, Metallica, and Slayer]. When we did the tour Clash of the Titans [with Megadeth] around '90, '91, it was always, you know, you couldn't really be at war after that point. We thought we were better than everybody. I would imagine Anthrax probably did too. Dave Mustaine thinks that to this day. [Laughs]
King is implying Mustaine probably still thinks Megadeth is better than Slayer and Anthrax. When Megadeth and Slayer toured in 2009, Kerry King joked that if Mustaine was going to be a dick to him, King would be a dick right back. King has also referred to Mustaine as crazy in past interviews, wondering how Megadeth kept any members. Most interestingly, in 2013, Tom Araya was asked about the possibility of more Big Four shows, and seemed to imply the main hold out was Dave Mustaine, who previously complained about Megadeth going on before Slayer.
Maybe I'm reading too much into this and these guys are the best of friends. Either way, great joke by Kerry King.