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RamenRider He has not tried to murder her for past few years, he's only sought to find her. You're assuming things based on his overwhelming dreadful appearance that was shown at the start. We're not sure that he really wanted to kill her, he would however exile or confine her away from society if need be to avoid riots and insurgency.
I don't know what kind of morals you're trying to apply to the story, but I can claim for sure the
Western Emperor has more malicious intents to do with her(both the maid and the Princess sides of the coin) than the Crown Prince. Ergo he wants to use her to bring down said Crown Prince and the Eastern Empire altogether, while the other just wants to keep the peace and people happy without complications and civil war rising everywhere to destabilize his country. The reason the Crown Prince needs her is because he's disfavoured by his father the Emperor, to gain the edge to overthrow their bullshit.
It's not like it completely excuses the Crown Prince for his behaviour or intentions, but at least he's a more decent person than the other nefarious miscreant playing games in foreign country trying to wreck havoc to his own amusement regardless of consequences.
The Crown Prince has the negative side that he just assumes he's being targetted by his enemies straight away in this chapter when its most likely just an accident though. But it's still an acceptable line-of-thought given his circumstance and environment. When you're surrounded by enemies and the monarch of your archenemy nation suddenly shows up uninvited, it's easy to fall into that line of thought.
P.S.
Stop calling the Western monarch a prince, he's the bloody Emperor of the Western Empire.
P.S.S. @
mitsuki48 Kiel is a push-over and we don't even know the depths of his character or history so he's quite dubious of a choice. Consider he betrayed his best friend(Crown Prince) and his possible lover(Crown Prince's sister) for unknown reasons.