oh lord i feel like he's got a badly misunderstood backstory
the 'person he killed' to obtain dark magic might be his mother, since he appears to feel deep sadness and somewhat regretful towards her. that was why he was still kind and lenient towards katarina, because she reminded him of her. i can bet the death was unintentional, or he was forced to, and it led him to feel guilt/hatred towards forcees and go down the dark path.
he doesn't seem genuinely evil. if the person he sacrificed was truly his mother, then he hasn't actually killed anyone. i would've thought that maria would be tortured or chained or something, but he simply put her unconscious. she did not undergo any physical harm.
plus there was that crying scene when he put katarina to sleep. he's bent on 'revenge,' so again, if theory 1 is correct, then he's seeking vengeance for his mother. he wishes to overpower the people that forced his hand at a young age when he was weak. seeing how he's bent on the high nobility circle, i think it's safe to assume his enemy is high up in the ranks, maybe related to one of our mcs.
conclusion: prez is going to simp for bakarina in the near future and i'm living for it