Is selling your soul to a devil an evil act?
Yes. He is now going to Hell when he dies, that more than anything else should give this away :P
Does anything else matter, circumstances, other aspects of the deal?
No.
The character committed an evil act, and it sounds like he committed numerous evil acts coming to this point. He should become evil, his own intentions be damned. He should probably become chaotic evil since all of his actions are largely chaotic even if making a deal with a devil is technically LE.
F!~@ if some serial rapist is going to betray his comrades and make pacts with Hell and sit pretty pretending he's neutral on the scales of Good and Evil. Boy hasn't toed the line, he's made a running leap, arched his back over the bar, and made a ten point landing squarely in Asmodeus's sitting room. Unless he saved no fewer than 36,432 puppies in the past year and found good homes for all of them he's not playing a neutral game, I don't care who your GM is.