
If Soo-won knew he was going to die young, why did he take over as king? If he truly loved and wanted to protect the kingdom he wouldn't initially have planned to die without a wife + kid. His original plan was to kill Yona. So like what was he thinking?

I believe we still don't know the exact motivation of Soo-won, but what's certain is the late King Il killed his father and then lead the country poorly that it became a mediocre nation. To realize his goal (we still don't know yet till now), the first step Soo-won needed was a position of power, and to do that he had to kill King Il (current king) and Yona (another royal heir).
Soo-won already planned this for years and had the intention to carry his means as soon as possible, which is understandable given the fact that now we know that he has an incurable illness.

I don't think Soo-won plan on marrying or at least having children. He don't want his kids to die young like him. Political marriage, maybe.

Didn't soowon mention he already had a successor in mind? SInce his illness will pass to his child without fail, he probably never intends to have children since (like his mom and unlike his ancestors) he thinks the kings blood is more a curse than a divine blessing. He probably rather spare his cuture children having to watch him die/live without him

It's not certain that King Il killed his brother. This is yet to be determined. Also, Yona doesn't automatically ascend to the throne if her father died. If that were true, she would have been crowned when it was revealed she was alive at the palace. Or at least Soo-won would have been in trouble of losing the crown. No, he tried to kill her because she was a witness.
I thought the girl who liked Jungho that committed suicide was actually Lilith/a trap to push Jungho to being evil all along, but I guess not cause she got reincarnated.