he urged the ML to kill the empress dowager to get "revenge for the both of them" but actually he it's just for his pleasure and own interests

accidentally killed the two secret guards ML sent to protect him from all the chaos happening, he kept it from the ML when they talked about everything in the second life

MC actually felt happy seeing ML self-harm for him, as if he was very pleased to see ML also dies for him as he did in the past life, I mean su*cide is not to be taken lightly and our MC here make his su*cide in the past life the reason to hold ML in the throat. as if ML didn't suffered enough when MC is still alive and suffered more when he died

he suffers a lot. That doesn't magically fix the relationship.

the dreaming of the past to see the partners suffering has been done in other works too but this one had me bawling gosh

Anything that involves a person carving themselves up to feed flesh eating snakes is where you lose me. It makes the sleeping next to a corpse for 12 years seem mild and that makes me physically ill.

ML knows MC's best and worst sides. To say that he fits the empress role is a far fetch. ML is a great emperor but sacrifices a lot of himself and others for the country. He's that annoying guy who will NOT communicate with you, not explain things to you, and merely expects you to be patient with no end in sight. Only towards the very end did ML feel like opening up that clam-like mouth of his to MC.

MC, who's an extremist, is completely unfit to be what is asked of him. He'll act like a bull when it sees red, absolute tunnel vision. He hates with a bias and loves with a bias. Pettiness and vengefulness cloud his judgment long-term and the impulsive toxic actions, like self-harm, gaslighting, victim shaming, and snarky sarcasm, only create dislike and distance.
Pretty much the ultimate petty queen.

ML's extreme sides overwhelmingly top MCs' after his death and after some plot advancement machinations, MC gets to witness the physical and emotional prices ML paid for losing him, which FINALLY calmed MC down.
Nevertheless not praiseworthy on both sides to threaten each other with self-harm to make the other pay attention or make them stay.
And it's only after seeing enough bloodshed from ML that MC became complacent enough to forgive him.
MC becomes more annoying over time with his hypocrisy though, reprimanding ML for self-harm but doesn't self-reflect on himself. He still used that trick to gain ML's attention, by grabbing a knife to scare him. How MC was able to laugh that off in his own mind was plain scary.
He did verbally admit to that fault but when he attempted to scare ML, it was after the reincarnation and after he found out the truth of their first lives, so it still doesn't make it any better.

ML too, remains stuck in his old ways by NOT communicating and often withholding information and lastly struggles to keep up the transparency with MC.

The backlash of the spell to return them back to the past on ML works out well as a reminder for ML to keep loving him, but the question remains if it will ever be the chains that bind him to MC or be just an obstacle in the future if their love fades.

There were plenty of irregularities in the story such as the empress dowager and ML: ML was aware of her evil nature but never bothered to warn MC properly nor explain it to him. It was said that he's filial to his birth mother but given the risks, his lover is exposed to near such a dangerous person at all times, which should warrant him to loosen that mouth of his a little.

In the later years into their marriage in their first lives, MC had become a very dislikeable person. Those traits weren't new either, as MC had often displayed his fits towards his siblings and father.
The new level of excessiveness stems from of course the outer force, the empress dowager, and his newly gained power as the lord empress.
In the novel, they kept repeating how MC was never such a person, but it's simply because nobody gave him the reason to.
His father could very well be a perfect trigger to worsen his character if he didn't love his son. Minister Yun always gave in to his son and his siblings are always careful around him due to his relapsing childhood injuries.