All he'd wanted was to move on with his life after giving up his failed experimentations on the SOUL's psyche. All he wanted was for things to go back to normal, where Mr. Black was no longer in the picture. He would, unfortunately, not get what he wanted. Mr. Black returned, and that was the final straw. He had to end himself to protect those around himself. The skeleton brothers will now have to learn to move on without their father. Alphys will have to continue her role as lead scientist, without a mentor to keep learning from. Monsters will have to learn to keep on without their friend. And Sans, he'll have to find out what Dr. Gaster's research was really all about, and why it killed him in the end.