At the beginning, the MC has been married to the ML for a century. She loves him much and she feels indebted to him whereas the ML is distant, giving her crumbles of care. The turning point in their relationship is the appeareance of another woman who captures the attention of the ML. The MC painfully realizes that her love is unrequited and asks for a divorce. The ML agrees but warns her that she is nothing for him any longer. Soon after, the MC discovers that her native house is in grave danger and goes back to beg the ML for help. He blocks her from meeting him, ignores her pleadings and refuses to save her when she gets kidnapped by one of his enemy. Heartbroken, she sacrifices her heart to escape at the cost of dying shortly thereafter.

From a moral point of view, this ML is undoubtedly unforgivable. He has married her with the sole purpose of cultivating and then taking out a powerful magical artifact inside of her. During their long marriage, he does develop bland feelings of hesitation and pity, but... too little, too late. From the ML's point of view, he has spent centuries looking for this artifact and, as his past, hard life hardened his heart and conscience, he doesn't consider the MC's life more important than his long-desired goal.

After the MC dies, she wakes up in another person's body in the past and swears to herself she will never fall in love or pity the ML. On the contrary, she should kill him to prevent him from becoming a powerful demon. At this point, the parties are reverted: the ML is the weak and pitiful one who gets attached to the MC as she gives him crumbles of kindness.

Overall, I feel that the MC is too much ruthless against the ML. She is justified, mind you, but still, she should realize that, being the past, her resentment is directed towards an innocent person. He is not the unloving, cold person of the future. He helps and saves the MC many times only to be deceived and betrayed by her uncaring attitude till the point that he

The ending might be a bit unsatisfactory for someone, it's an open one.