I feel like my conspiracy theory that Su-Won is actually another Dragon God that came down in mortal form is less mental. A 'psychopath' wouldn't calm people's anger down, they would fan it, and wouldn't give Il a ...10 years? observation of 'being fit for being king' and then a clean stab to the heart for a death, nor would a child who's upset think to do so. He's also just acting far too mature and thinking too many steps ahead for his age.
Also: We don't know that Yu-Hon killed the queen, nor do we know that Keishuk's info was true. The first was just flat out never shown and Yu-Hon pretty proudly owned up to the massacre, so why not the Queen, and the second was shown as a recounting from a super sketchy guy.