in general, everything depends on the player's choice; in a map for Heroes 5, Dark Messiah has 2 endings, only 1 of which is suitable, and in the game Dark Messiah, there are 4 endings, and 2 of them are suitable. I believe that the "good" ending with Zana is the most logical.
1) Sareth himself says that he enjoys transforming into a demon - personally, I see this as him accepting his true nature and is unlikely to repent.
2) judging by Sareth's reaction to the entry in Arantir's diary, he is against the mass murder of innocent people, which means he will not release his father.
by the way, Sareth could then be given the Zehir specialization (master of elementals) because demons are in the place of elementals, so they will appear in the place of the deceased squad, and not elementals.
