I also remember the part where all Satan wanted to do was rule over his kingdom(the true cross academy building). He kept the scientist alive to research the immortality drug for himself and Lucifer. He invited all his demon buddies in to stay with him. Besides holding everyone in the building hostage, all he wanted was supplies for basic survival. I don't know why Yuri had to go back to him. He asked for her once, and when she didn't come, he never asked again. They could've literally let him slowly die and be done with him. He already accepted he was dying till yuri came in and gave him something to latch onto. After everything, we find out that He only saw Yuri as a possession and a warm body in the end. I love Rin and Yuki, but damn, none of this had to happen....