And this is so true. Sephiroth has (or gained, he never came off this way while he perceived himself as human) malignant narcissism, which outlines all that he does and perceives about himself. He sees himself above all others and carries within him an unyielding maliciousness that manifests through murderous, sadistic rage. However, it's a cold, calculated and controlled rage that doesn't carry classic emotional outbursts, it's just a smoldering hatred that guides Sephiroth to commit the acts murder that he does.
However, the grievous injury and subsequent defeat he suffered by Cloud hurt him. It hurt him bad. He suffered what's essentially called a narcissistic injury that devastated his own perceived invulnerability and superiority. He felt fear, humiliation and shame.
Sephiroth had never been hurt, pushed, or caught unaware before that night. The night he declared himself a god and the chosen one to rule over the planet, he was made to feel the most vulnerable and human he's ever felt in his whole life. He went from feeling omnipotent and full of pride to being so weak he could only carry his mother's decapitated head with him. He then was subsequently tossed into the Mako Reactor wall, blown up, and he proceeded to tumble into a Mako Pit where he died.
That colossal humiliation would not be a slight a narcissist like Sephiroth would move on from. The entire realignment of his perceived invulnerability that Cloud inflicted onto him, has to be paid back and acknowledged. For Sephiroth to peacefully exist and reconcile his perceived invulnerability with what happened to him that night, Cloud has to suffer. Cloud has to break. Cloud must be made not just be smaller than him, but minuscule and beneath him in every conceivable way. For Sephiroth, that hurt he felt in Nibelheim has to be paid back with interest. Only when that happens will Sephiroth move on from him.
Let's just hope this is all there is to it, and not Sephiroth being pissed because of his 3 defeats in the Nibelheim Mako reactor, FFVII, and AC(C).