Responses
Yes. The prince was probably devastated after killing the hero and the demonic entity might have took that opportunity to influence and curse the prince. We know that even he was influenced like the hero and became a tyrant.
I have a wild theory about what actually happened in their past lives, especially considering that latest flashback.
Most likely, some demonic entity cursed the prince, and the hero somehow ended up taking that curse onto himself.
It all ended with him slaughtering a bunch of innocent people, that forced the prince to stop him aka kill him.
Or perhaps the hero was the curse's original target. In any case, that flashback shows him killing a multitude of people while clearly not in his right mind; Talon and the prince react just as any rational person would -completely baffled by what is happening.
But given the earlier flashbacks, I still suspect it was a curse that affected both of them...