#look.....

I think he would've become a knight to please his father and maybe gain some sort of respect or acknowledgement since he's a bastard child. Receiving a quest to brave the most horrible place known to the kingdom and re-establish contact would be something he sees as a chance to gain the acknowledgement he's been craving.

Obviously, when he gets there, he immediately knows that shit is completely fucked but it's too late to turn back (and if he does, he'll never be respected) so he decides to investigate what went down.

He would have relatively low sanity by the time he runs into Prometheus and the Girl in the Mines. He nearly attacks them but comes to when he recognizes that one of them is a child. After that, he would join them on their way to Mahabre and probably lose an arm to the Crow Mauler in the process. By the time they reach the Tower of the Endless and Nilvan asks them to take the Girl to the Altar of Darkness and Prometheus says "Yes," he knows he's going to fight tooth and nail to get the child as far away as possible from those fucked up dungeons.

I think he'd fight with Prometheus first, arguing that a child shouldn't bear such a curse. Prometheus, being the Dark Priest, would be much more pragmatic and suggest that if Heracles finds it so upsetting that he could leave the dungeons and forget he ever met the two of them. Heracles would get angry and the two of them would start fighting, causing the Girl to run off and nearly get captured (and eaten) by a Lord of the Flies.

After rescuing the Girl, both of them would look at each other and think "fuck this" and take the Girl and leave. I think that for a while after they leave, Nilvan would haunt Prometheus's dreams and torture him with nightmares for breaking his promise and forsaking humanity but her influence would fade with time and distance.