In terms of a metric akin to "mathematical DALYs", Galois was perhaps the most impactful early death of all, followed by Abel. Both Galois and Abel founded entire branches of mathematics before the age of 25.
Good question, maybe he shouldn't be on that list. I'd read a long biography about him in German ("Wer ist Alexander Grothendieck?"), and was left with the impression that he was unfulfilled and unhappy at the end of his career; of course he lived to old age and there were certainly worse fates on that list, so perhaps its inclusion is not so appropriate.
