This outcome was within Roy's expectations.
As a mature demon, it was another matter if he was evil or not, but he had to use all the resources and cards he could use to fight. He had been in a deadlock with Kha-Beleth, using harassment attacks and fighting guerrilla warfare, in order to force out his ultimate move and then use the Isabel card.
This was because Roy knew very well that demons, humans, and even angels, titans, and dragons were all the same. All creatures that used magic power to release magic had a common problem: the stronger the magic was, the more likely it would cause a magic power backlash when interrupted rashly. This was due to the nature of magic.
