Well, by the Ars Magica RAW, binding the demons using Infernal arts is already mortal sin. (Let's leave Solomon out of this for the time being, thusly.)
Anyway, the point I was trying to make was that it would make an interesting character to have someone who was morally virtuous (using the definitions of moral philosophy, and in having virtuous Personality Flaws) and theologically using infernal powers and thus committing mortal sin. Could the guy have turned to God for help and become a saint instead? Possibly, but clearly God didn't answer his prayers; he either lacked the necessary Faith, or God's plan was that the region suffer/his die rolls sucked, or as you noted, he lost faith in Heaven and decided to use the power that was there.
So you get someone who is both "good" and "damned." Obvious story fodder, especially if Hell can resist the temptation (ha ha) to give the guy Vengeful Powers.