I think I get what the problem here is. The author is too heavy-handed.
This is the oooooold trope of "cold-hearted man who warms up to a frail, gentle girl". Stories like this are a dime a dozen. But this author, like a few others, wanted to spice things up by making the male protagonist a cruel, blood-soaked, heartless warrior who condoned sadistic acts of murderous misogyny towards defenceless noble girls. They tried to window-dress it by making it look he killed that princess to save her the worse fate of being raped and humiliated before being executed anyway, but it fell flat.
And let's be honest, these authors don't give a damn to whether the setting makes sense from a moral standpoint. So long as the lead male is good to the girl, and shows some cheap notion of integrity, they can burn the world for all they care. This Elpasa Empire he fights for is clearly the aggressor in the first chapters. He might have stupid notions of achieving peace by unifying the continent. But it's bollocks. That becomes evident just by looking at countless series in which the heroes are actually the underdog country desperately fighting for their freedom from a conquering nation whose leader's ambition is… why, to unify the continent and ensure peace! They're self-evidently described as villains, and yet it's the exact same scenario as here, with roles reversed. Broken Blade is an example; Rain is another.
If this country still has borders that need this nonsensical ceremony to be protected, it means the goal of "long-lasting peace" hasn't even been achieved.
Cruelty is cruelty. Spitting on a man's face shouldn't warrant execution. War prisoners have rights. Etc. etc.
This guy is a murderer who sent many young foreign soldiers to an early grave and made many people miserable. His harsh childhood and feud with his father do nothing to relieve this. Chloe has every right to be irked by him.
But of course she isn't, because he "is harsh to the strong and kind to the weak". I wonder which the murdered princesses were. This story is made just for the sake of romance. It's not a big deal, others are, too, like This is obviously a fraudulent marriage where the male lead is basically a psychopath.
I guess the idea is that the more of a beast the male lead is, and the more vulnerable and at his mercy the female lead is, the greater the effect when she "tames" him. But if said taming is limited to their household affairs and life together, and at work he's still the same bloodthirsty maniac, well, it's still cringeworthy.
Sometimes I worry the authors underestimate the intelligence of their target audience. Do teenage Korean girls really think like this, dreaming of being embraced by Bluebeards who only they can control? I doubt it.
Still, the art is pretty, the female lead is resourceful (a hypocrite if she ends up falling for him despite everything, but who cares) and who needs depth and nuance when reading a brainless love story? They are welcome, but one can't wish for all authors to be gifted at plot and characterisation. Oh well.