The fact that he made the decision mostly due to a literal cursed seal on his neck which had a little hunk of pedo snake's soul in there chewing away at his mind?
Seriously, it is part the curse of power (side effect of the sharingan and the uchiha culture), part him being a reincarnation of a guy who believed might makes right, and part Pedo Snake's cursed seal.
This combined made him driven to seek power above all else, including abandoning those he cares about and joining up with an obvious psycho and his gang of demon children
1. You still don't provide examples of cruelty.
2. Even if his decision was influenced by a chakra construct it doesn't mean the change in his character. I recall how he insisted on non-lethal fighting, for example, instead of just killing everyone in his way.
3. What does the curse of hatred even have to with this? Any sane person would want justice to happen to the killer of their family.
4. Your last statement is blatantly false, seeing how he spared Naruto's life in the Valley of the End even though he could have gotten the Mangekyo Sharingan that day.
5. Obvious psycho? Orochimaru? How come?
Edited. I must clarify, that sounds wrong. Orochimaru's a criminal, a political enemy to the state, that's true. He's also cruel and vicious and callous. Also powerful, influential and overall successful self-made-man who dreams of becoming immortal. Who also experiments on people for science and collects interesting 'specimen'. Also founded a new hidden village (the only one to do this in the series btw) and was willing to provide tutelage (unlike Kakashi). He's a controversial figure, not a psycho.
3*. Not counting the fact that Curse of Hatred has never been confirmed to be a thing by an Uchiha. It's first introduced by Senju Tobirama, the guy who's obviously biased to the point of racism to the Uchiha, and later is confirmed by SoSP, who's also biased towards his elder son imo. How much of that is actually true? We wouldn't know, there are no Uchiha left to find out.
3**. A point could be made that Curse of Hatred is a real thing aka a brain illness and only happens once Mangekyo's achieved. But Sasuke doesn't have MS either at that point.
