Khan was run off of Earth because they (the supermen) lost. He had a score to settle with Earth and likely would've went out in a blaze of stupidity.
You mean Khan is the kind of guy to hold a grudge?
lol, true. But at that point everyone who has ever opposed Khan in the 1990s is long dead, what would he want with the planet itself?
Plus, Kirk really pissed him off. He never bothered to check their progress.