Luca isn't just a bully. He's the son of a mafia don. He's part of the mafia. He's a psychopath. He was made to shoot his own mother in the head when he was 12. Quite frankly, it's a miracle he's not MORE fucked in the head than we already see him. He's not a bully that failed to grow up, he's a monster.
His hatred of Harry isn't as random as it seems at first either. If you go through all the storylines and endings, you find out that before he started on the MC, said MC told him something that was almost word for word the same words that his mother told him right before he shot her in the head. This wasn't just random bullying. Luca utterly hates Harry because Harry triggered him with his most traumatic childhood memory. Said memory has such a strong hold on him that when talking with Laura at one point, he nearly chokes her to death because that's how much it pisses him off to remember.
Which is actually fairly normal. Childhood trauma of that kind can ruin people for life sometimes... and that's extra likely for someone like Luca who lived in less than pleasant environments with a father that only saw him as a tool.
As for winning... Well, it's a NTR plot. If realism was invoked, 99.(9)% of NTR plots wouldn't work no matter how much you squint at them. You have to turn your brain off to some degree or another to enjoy NTR... and even then, I'd say that this VN has done a way better job of the story than many others I've seen, most of which I don't even care to remember at this point.