I think it's really easy to blame Harsen, but if you stand in his shoes, he really didn't have many options after he chose to marry Ercella.
Late Duke Berhardt:
- Was against the marriage because the Visardes have a lot of enemies. Warns Harsen that he'll have to spill a lot of blood to marry her. (Also pressured Harsen throughout his life because he's illegitimate.)
Late King of Grania:
- Wanted to marry the princess to Harsen. Because he declined, sent him off to war just after his marriage as petty revenge.
Death of both fathers:
- Both the former Visarde and Bernhardt dukes die around the time Harsen is at battle. Keep in mind, Harsen is barely an adult at the time, and his similarly young wife is suffering from depression and fending off usurpers while he is away.
Current King of Grania:
- Wants to weaken both Visarde and Bernhardt as vengeance for his late lover, and fear of their growing power.
Duke Caron of Visarde: (Ercella's brother)
- Uses Harsen and Bernhardt as a shield against Visarde's enemies. Makes Harsen do a lot of dirty work while threatening him with Ercella's divorce.
Duke Garten:
- Harsen needed Duke Garten's support (the haughty girl Vicente is engaged to), because he was being used as a dishrag on multiple ends. He can't really ask his wife for help either, because women in Grania aren't respected and have no real power. Duke Garten convinces him to rebel, and he ultimately decides to coup because he's tired of doing everyone's dirty work while having a noose around his neck. It doesn't end well.
Regardless of whether Duke Garten convinced him or not, King Grania was going to cut him off anyway.
Harsen has been in mental wars since young, and real wars since barely legal age. I don't think his motivations are great, but there are a lot of political-situational factors in play. He was effectively shouldering two households on his back. He can't even peacefully be with the person he wants the most, when she needs him the most, and whom he knows doesn't love him back. A lot of things are just eating at him inside.
So yeah, I do think he's a terrible person, but I also pity him. If he hadn't chosen Ercella, they both could have lived with (slightly) less bloodshed.