Well it actually happened! I actually killed a character in 4e, and not some low level but a 15th level wizard. Here's how how it happened:
For some weird reason all my friends highest level characters seem to be wizards. So here they are running through one of the new second season Living Forgotten Realms high level modules (to those unfamiliar with LFR it seems after getting their feet wet with softball mods the LFR writers have stepped it up a notch with killer mods) with a party composed of three wizards and a druid/assassin. They are in the last encounter fighting a solo elite with a couple throw-away 100 hit pointers (against the wizards 100 hits will last about a round and a half). Luckily the solo elite has a few trick up his sleeve, like an interrupt that teleports players attacking allies next to him, a recharge on a 4,5,6 6d10 encounter attack, and a +4 vicious weapon that does an extra 4d12 on crits and action points (Trust me an actual 15th level player swordmage is going to have all this and more). So after the wizard attacks one of the 100 pointers, the elite solo whip the wizard out to the center doing a bit of damage, and hits the wizard on his turn with the encounter, at which point the wizard is down to 2 hit points. Being the kind of DM I am and having played 4e long enough to know that most players can come back from 2 hit point to maximum in a single round, what do I do? I action point. What do I roll 20!? Well the wizards not dead until minus -42 so there's still a chance as crits base damage is only 36, but then the extra 4d12 get rolled 1 point, 2 points O.K. so far but then a 9 and a 12 for a total of 60 points. Game over! Of course 4e players are not used to death so the player storms out of the room complaining about viscous DMs, Evil adventure writers, and claiming he's selling all his 4e books on e-bay never playing again, before his fellow adventures can explain they have a scroll in their pocket that will bring him fully back, or the druid/assassin can crank out three 100 point crits in a row to bring the elite soldier down.
I think I played the encounter as the rules allow and the writer intended, but having angry players always leaves a sour taste in my mouth. So what do you think? Should I play the dice as rolled , or fudge the damage so he is at -41 unconscious but still alive, or not have used the action point in the first place? She-who-must-be-obeyed points out that since a mere 3 points would taken him out the crit would have been far better spent putting 60 points on the druid/assassin and represents a total waste of 57 points, but that's just the way she rolls.
P.S. Due to the fact that this is the weekend for Con-on-the-Cob, level 1 of the Ruins of Thetra Mage college may not appear until next Friday.