bite the dust, buy the farm, check out, cross over, expire, succumb, give up the ghost, pass, pass away, pass on, be no more, cease to be, go to meet one's maker, be a stiff, push up the daisies, hop off the twig, kick the bucket, shuffle off this mortal coil, join the choir invisible
It's interesting that three contributors have mentioned 'bought the farm' but nobody's mentioned the allusive abbreviation (which I associate with RAF pilots, particularly in the Battle of Britain, but which has remained current in Br English: 'He's bought it').
A euphemism that I met in the '70s, when I hitched a ride with a mercenary, is 'account for' (which means kill not die).
(Irrelevant reminiscence: He didn't say he was a mercenary; he said he was a soldier. But when I said 'It's not every soldier that drives a car like this' he said 'It's not every soldier that fights as hard as I do'. I didn't pursue the matter. )