We haven't seen the mark on the delinquent guy who's supposedly his majesty. This is only my theory. What if, the black hair guy is the actual Sookbin? What if he did some sort of ritual to make them regain their memories and made the teacher and FL misunderstood? I know this is silly because the black hair guy also mentioned about taking revenge together. However, we haven't seen the back of his hand where the scar should exist while we have seen it on the teacher.
OR what if that is really how the king was in reality and the 'kind and gentle side' was only shown to the queen back in their past life. Which may also be why the concubine rebelled and killed both the king and queen because the king did something and the queen was unaware of it but may have been unknowingly complicit...or something hahaha.
Plus, they did say he had a weak body back then, which might have been why he played it safe by going the gentle route since he needed all the support while hiding his black side behind the mask ╮( ̄▽ ̄)╭
That evil twin is fucking smart though, I have to admit. He sees an opportunity and quickly take it.