Hellstorm is both good and evil. He does still rule part of Hell, just not as much as Mephisto. Just about any demonic entity can get a person's soul. And what person wouldn't want control of their ex-spouse's soul?
When Damon first appeared, during the day he was Damon Hellstorm. At night his demonic side mostly took over to become super-D*%& The Son of Satan, but still pretty good...just a jerk
A few issues into his run on Marvel Spotlight, the Damon half pretty much took completly over until Warren Ellis' excellent Hellstorm book. He killed Dr. Druid and his change helped Patsy commit suicide.
Until the Max book he was rarley seen. Lately he has returned to being super-d*&%.....