Id say it depends on how he becomes a demon. If he goes through the normal soul extracty process that we see in eternal i see a few different outcomes:
A) Kalibas would deem him unfit for soul extraction process and send him to the blood swamps, in which case he’d go back to razing hell (which I honestly find really unlikely since someone like him would probably have a lot of soul power)
B) it doesn’t work, they’d just torture him forever because his will is too strong, he might even escape and return to his crusades.
C) it does work, his will would be completely and utterly broken, his soul would be extracted and he’d sit there for thousands of years slowly rotting into a mindless demon drone.
However if it was through some magic bullshittery that only changed his outward appearance then yeah you’re absolutely right, he’d just keep killing demons
