"I hate the King of Kepler. I want him dead, gone, erased from history, his name forgotten. When I'm done with him, there won't be a Kepler left." is a perfectly legitimate, IC sentiment.
"I hate the King of Kepler. I'll ask the Count of Kepler Fields to swear allegiance to Evilstan, evince him from his manor, then come back to Kepler" is not something a medieval noble would do. It's meta-gaming.
Well... actually... I tend to think of many medieval nobles as lawyers. Finding a loophole in the laws to get their way, if need be. You only have to read Game of Thrones to get that.
