I would've said the Little Goomba mushroom in the original Super Mario Bros. until I remembered my mother getting killed not once, not twice, but THREE TIMES before she finally figured out how to measure her jump to land on it. She was then promptly killed by the game's second enemy. This was in December of 1988, which means my mother can now celebrate twenty years of being a non-gamer because "I tried it but I just don't have the reaction time of you kids." Let the record show that in May of 2010 I'll be the same age as she was at that point and I sure as hell don't need a kid's reaction time not to get killed.
Now then, with that anecdote told, my choice for Easiest Kill is Metal Man in Dr. Wily's Castle in Mega Man 2. One shot from his own weapon and it's all over.