Seriously, no death game is complete without a guy driven mad with guilt over the death of a friend. We sort of had that with Kirito when the Moonlit Black Cats died, causing him to never join a guild as to not suffer the same loss again
Also quick fan theory, remember the revival item that Kirito gave Kielen, what if that was what saved him. Remember, when Heathcliff stabbed Kirito, he survived a split second longer than he should have, allowing him to kill Kayaba. Think about it, if Asuna's love for Kirito was enough to break HER paralysis, is it so much of a stretch to assume the same of Kilien, seeing his first friend in SAO, the one who taught him how to survive, almost die? Maybe the revival item brings you back on one health, which is why it didn't look like any health was gained, and