@nv17 I thought that for a long time too. At the same time, the state of the nation kind of needed an urgent change of power too - for example, I think it's been reluctantly acknowledged by some characters that under King Il's reign, the nation wouldn't have survived the Fire Tribe's attempt to usurp of the throne. Then with the problem with illegal trade of drugs from the Sei (?) empire, if they didn't stop it by force then the drug addiction would have likely spread throughout the whole country.
I don't think what Soo-won did was right, but maybe he got desperate?
Although, Yona also proves that even without a powerful position, you can handle these problems to some extent (just have some strong people who are loyal to you). Although there is also a limit to how much you can do without power.
Or maybe he could've tried gaining a following for his cause and grow in influence without the need to murder the king.