I guess it depends on how you're adding those 50% together: add three bullets to a 6 chamber revolver for a game of Russian roulette and 3 bullets = 50% chance of death. If you take two tries, sure, that's not 100% chance of death. But 3 bullets = 50% chance, so if you add another another 3 bullets then those percentages combine quite well for 100% chance of death :)

(assuming your aim is good & lethality on hit is 100%)