Just as a side-note, I know that you said this wasn't critical hits, but people in-game sometimes die at random times for unknown reasons, usually when one's under 4 hearts. A couple critical hits with a diamond sword shouldn't kill a diamond armor person, but the death may have been partially due to the insta-death glitch, as well.
(3 critical hits with a Diamond sword would not be nearly close to killing a diamond-armor person, though)
