Actually, the gaster blasters are canid skulls, meaning they're dog skulls and not dragon skulls. Also, you don't kill everyone- there are a few that get away, like monster kid, Alphys, Bratty, and much more.
Fighting Sans; Fighting a skeleton with a glowing blue eye that is wearing an eskimo jacket and khakis, with two pink slippers and socks that are being worn with the slippers, that can teleport, summon dragon skulls that shoot laser beams,that can yeet you across walls, and can also yeet you against walls and summon bones out of the walls, that yeets bones at you that has 1 HP and is fighting you because you killed everybody in the underground.