I've been reading recently that maybe Suwon didn't killed Il, but like your opinion, i don't think Sensei will trick us. First of all, why'd he bring his sword if he didn't intend to kill him? and even if let's say Il summoned him, regarding he believes Il killed his father, bringing his sword as for his protection, then what he asked for his soldiers to prepare? it's still a planned coup! i also don't think Il asked Suwon to kill himself and leave Yona in an instant.
I believe either Soo-Won really did mean to kill Il from the start or he was manipulated by someone into doing it (and lied to about Il killing his dad). The fact that other characters noted Yona’s ability to work with Soo-Won despite what he did is, I think, further proof that he really murdered Il.