Something that I think is pretty fun is that you don't necessarily have to do too much to characterise historical characters in your writing
Recently I developed a character for a House my little sister developed in the lore of our story. It's actually one of my favourite pieces of lore in the entire story and they're one of the most fucked up white haired families since the Malfoys in Harry Potter and the Targaryens in Game of Thrones.
But the character I developed for the House was a 13th Century knight who fought for the Anglo-Norman forces against the Irish and became the Earl of Leinster after defeating the head of a rival Irish Clan. Then when he came back home he was spurred on by his Earldom, desire for land, and the Catholic Church's opposition to consanguinity to kill off a sect of the House that was practicing incest, and he blooded his children by getting them involved in that battle.
So, overall, a pretty fucked up guy even when dealing with something that we can all agree is pretty fucked up. He prevented the House from becoming the Targaryens, but at what cost?
But the characterise him, I didn't need to do a lot of work to be honest. Besides noting that he was extremely arrogant and had nice hair, there was one bit of characterisation I gave the Earl of Leinster that basically sums up his whole personality.
During The Battle of The Frozen Bogs, which is the battle where he fought against the Irish Clansman, I imagined him in his knight armour on an armoured horse. And as he gets ready to fight the Clansmen he snaps his visor down and lets out an arrogant "Ha-ha!" and then charges in on his horse. That says everything you need to know about his character in a single action and I think that's pretty funny.