100 years ago the emperor fell madly in love with a young blue-eyed architect. His love was not normal, he wanted to cage him, cut off all of his escapes, and possessed his entire being. He was insecure that the man didn't love him back, so he tied him down by being abusive and killing countless people who stood in the way. As twisted as it was, the emperor was so blinded by his own love/lust that he failed to notice the young man has loved him too, from the beginning, but it was too late for them and the young man ended his life to stop the blood shed.
100 years later, they were both reincarnated, the young man has all of his past memories in tact. The emperor fell for him this time, but it traumatized the young man who resisted with all of his might, fearing history will repeat itself. The reincarnated emperor acted differently this time, desperate to not repeat his mistakes and lose his love. He's still madly in love, but he learned to control himself and put the young man's feelings first.
Palace of Bardo