
The power and arrogance got to his head. The empress's family is a powerful one who was behind the selection of kings. The trash is of illegimate bloodline and it was because of the empress he becme emperor. He knew this and instead of being grateful, he became fearful and decided to dispose of her family in case they change their mind.
He delusional enough that he powerful enough to get away with mistreating the empress thinking she has no one and is incapable of anything by herself.

Il a un complexe d’infériorité. C’est le bâtard de la famille royale, et elle, elle vient d’une famille tellement puissante qu’ils peuvent même mettre un minable comme lui sur le trône, et ça touche son égo. Il se méfie d’eux et en même temps il se sent inférieur alors que c’est lui l’empereur. Mélange ça avec un peu de sociopathie et il fait du mal à la femme qu’il aime, mais il ne sait même pas ce qu’il ressent.
Le rythme est rapide par moment oui. Surtout le dernier chapitre avec les retrouvailles de la petite.

It showed his thoughts early on. He had the thought that if they could make someone ask lowly as him Emperor then they could make anyone Emperor. He decided to kill them because his throne was at risk. Once they die and she's vanished he says "now I'm the true emperor", thinking there is no more threat to his rule, but he's still crazy paranoid about the FL. She just said that even her doing nothing and staying inside is too much for him to bear. He just sent someone into the house for that reason, he's terrified of her.
He's insane, but there does seem to have been a moment in time where he loved her. It just became overshadowed by his fear of her threat to his reign and by her own hatred of him that led to her denying him at every turn. Which he pretends doesn't bother him, but clearly does.
I don't understand why he hates the Empress so much, she seemed to like him at first so I don't understand his hatred towards her.