Worst way I saw was in a college 2E game. New player shows up for the first time, and spends about an hour building his character to be about our level. Once done, the DM setup his introduction as a typical "meet at the inn" scenario. Two of the players decided to haze the new guy (we were a rather contentious bunch), and ambushed him when he left the inn. They threw a bag over his head and started to pummel him, but the very first hit was a natural 20... and the DM just got a critical hit/miss chart from somewhere. After three dice rolls, the DM determined that his neck snapped, killing him instantly. Needless to say we never saw that guy again, and we came to HATE that chart over time (dismemberment was VERY common).
My own worst death was my very first character back in 1E. For those of you unfamiliar with that edition, the rule were not in the PHB, but in the DMG. Players had to learn the game as they went, knowing only about their characters along the way. My brother decided I was an annoyance and had his illusionist challenged my paladin to a PvP battle. The DM set the rules, and my brother opened the battle with Phantasmal Force. In 1E you had to actively disbelieve an illusion, of which I had no idea. His illusion was a mirror of myself, with him spending his action to have the illusion copy my movements, making it a coin toss between me and myself. His illusion "killed" me, droping me unconscious, where he then proceeded to stroll over and slit my throat. Bad enough to die in such a way, but having my brother lord it over me for months was worse.