Hello, I've made one of the chapter's the antagonist's POV, right before a big battle. I wanted to show a bit of how his kingdom and people lived, since we've only been in the protagonist (and his allies)'s kingdom and culture since the start. While I've done that, I'm finding it hard trying to justify their attacks on the enemy. He is the grandson of a man who came to Britain to "help" a British king before slaughtering every single politician of that King's government and declaring himself king instead. The antagonist and his brother both think of this usurper as a hero and great man. I'm really not sure how to portray the antagonist's emotions and thoughts regarding the enemy.
A Shakespearean hoax was made about some of these characters. I looked into it but it was so badly written that it was of absolutely no value whatsoever.
Deal with him like with any other MC. Make him real, give him feelings and cares. And things to fight for.
But... That's what I'm asking. There's not really any reason for him to fight, which is what I'm trying to find.
Then give him a reason! You are the author. If he doesn't have a reason then he will not fight, simple as that. This touches not only on character building, but also on world building. Think how the people are living in his country, if he is content or not how it is. Think how the different nations relate to another. Politically or otherwise. Trade, justice, there are a myriad ways for them to be at odds. What would he like to change, is he jealous, or whatever?
I'm writing historical fiction, so at this point he's fighting just because he did. Their people are quite poor and don't eat very well. But another antagonist already has that reason, to give his people more land and more food in order to prosper. It would be boring to use that twice.
Then give him something more. Perhaps he just really believes in the ruling style and laws that came with the usurper. I have an antagonist Apharael who is essentially your usurper grandfather. He came, he saw, he conquered, and he believes things are better with him in charge, because he's make things very productive and ordered. Or perhaps the grandfather was murdered and he and his brother want revenge for the "betrayal" of their family by their conquered subjects. Maybe he's just a selfish person, and cares about himself and his family above everyone else. You don't have to make character believe in some greater good to make them sympathetic, they're are a million ways to be sympathetic. I also think it's important to note getting stuff for your faction was a more socially acceptable excuse for violence in the past, and violence was more socially acceptable. World War I was fought over land and prestige. Some people wanted more(Germany). Some people wanted to reduce their enemy's to protect their dominance(Britain against Germany).