After genocide route, W.D. Gaster has found out about your killing Sans and Papyrus, and is angered by your actions. He knows you'll come to kill him, so he sits and waits... and practices using all the different soul colors! Once you come to kill Gaster, he is much more prepared to fight than you ever could've imagined he could be, and you are hardly ready for a real fight...
A big hip-hip hooray to pococikapusta for making Undertale Simple Battle Creator v 3.3b.
