What was Yu-hun thinking?

One question I seriously ask myself is why Yu-hun wanted to kill Yona and her mother.

First of all, I think he is a pretty disturbed person!

But obviously his family, especially his brother, is very important to him and he didn't want to be on the throne either.

So what did he hope to achieve by the death of the two?

One explanation would be that he hoped that the red death would disappear with him. (Based on the statements: since I took you as my wife, I hate the red dragon king more and more every day. May he be the first king! May he be a god! I won't let him take away the thing I love most in the world!)

Nevertheless, that doesn't make sense. He lived for another 2 months after the statement (he said this directly after Yona's mother died) and didn't seem to have tried to hurt Yona again.

Yes, he believed that Kashi was manipulating his brother, but then why kill Yona too.

What's more, he can't really believe that it could change anything about Il's leadership style.