The problem is that she can't cool down. She's not insane enough to follow through with Mizari, but she is antsy enough to immediately believe that Yona is dead once she leaves the castle. Hell, even when Yona says that her relationship with Soo-Won is on shaky ground, Kouren still interrogates her. On top of that, she wants to torture the dragons to see if they are alongside Soo-Won, and if you looked at history on uncovering info via torture, you'll know that it's completely bias and people usually lie to end the torture. Yona states that she had nothing to do with Soo-Won even after, and yet Kouren is trying to dig into her private feelings of if she hates him or not. That, to me, is paranoia. Paranoia is one of the leading downfalls of any kingdom as shown in history, and even the most level-headed leaders succumbed to it. Even if Kouren was level-headed, she would have probably created a Kensington System on her sister to make sure that she wouldn't be a conspirator.
WTF!!!! They were trying to kill kouren!!!!! I don't exactly agree with her, but I still think that she will be a great ruler once her head cools a bit. So seriously, WTF??????!!!!!!! YOU'RE GONNA KILL HER OFF BECAUSE YOU CAN'T CONTROL HER?!?!?!?!?!?!?!