If I try so change a score of a dead player it doesn't work until the player respawns. I can't set his score, can't add something to his score or remove something from his score. But I can execute something from the dead player, ex. I can summon an armor stand with the /execute command on his position. Put if I can't reset the score of the dead player there will spawn a lot of armor stands, put I only need one at the death point of the player.