Ok, the princess must have done something to him because that was just plain hate. I don't want to excuse the ML actions, but maybe there is a reason? Like, maybe the princess did something horrible to him and when the ML send the letter asking her to marry him they instantly thought of MC because she looks like her. Sacrificing the MC in order to keep the princess alive.
If the princess really did nothing then I don't understand because he seems to know her, he even imply she could have brought a weapon to kill him because "she" hates him...
Oh well, we will know latter about this.