I need a motive to kill a princess
I'm still working on the basics, bit this is what I got so far: the princess lives under the cruel reign of her grandfather and when her brother is killed, she is forced into a quick marriage so that there is a male heir. She doesn't like this and makes a plan to join a fighting arena masked, reveal herself when she wins and so proving to the king (a warlord so he values physical prowess) that she's capable of ruling alone. There is also a civil rebellion on the rise who are fed up with the cruel king and send an assassin disguised as a concubine into the palace (who is the eventual love interest) who is supposed to kill the princess. There is also supposed to be a pivotal moment where she's unmasked in the arena too early and the crowd screams for her death and she's trapped in there with a bunch of wild animals or something.
What I still need is a motive for both of them. I thought maybe that she's known for being as cruel and violent or even more so than the king and the rebels is afraid she'll be even worse when she gets power. I could handwave the crowd screaming for her death for the same reason, but why does the owner of the arena allow it? Another factor in the story is a count who killed the princess' father and brother so that his son, who is to marry the princess, is put on the throne.