
I think they had an agreement, thats why Garon didn't kill his brother, Raon(?). Type kind of sht like, maybe they have some scheme going on. Raon offer Garon things that would make him a superior king, idk what but maybe military strategies or maybe how Garon can gained influence using him. So Garon saw Raon potential and says, "Yeah, okay. But if you even THINK to do something that crossing me, you are dead."
Idk about him and how he related with the MC's mom getting killed to be specific. But, i got the feeling he got something to do with the village getting attack and nobody prepared. Like maybe Garon send Raon to do some dispatch through the village, so maybe it is not specifically with the MC but maybe related to Imae as a whole.

The type of person the king is, he would've never let Raon live unless he was able to convince the king to keep him alive as you said, but the potential for him to cause a coup and take over the throne, or have children who will cause issues later to take over the throne is too high for the king to have ignored. That's why I'm saying Raon isn't to be trusted purely by the fact he's still alive.

I agree with you in the part of why Garon let Raon alive theory. What i did not agree is the theory that Raon have something specific with MC for the matter of the massacre of the Imae village. He may have something to do with it but not specifically because of the MC. Because all of the people are gone and just a few that still alive and breathing, the MC was just got lucky he wasn't at home. That's it hehe :D
I STILL DONT TRUST SML AT ALLLLLLLLL. He's related to the king, and the fact he's alive when the king killed off the rest of his family is a massive red flag to me. There's no way the king would've left anyone alive to dispute his rule or create potential heirs that could cause trouble later, so SML would've manipulated the king somehow into leaving him alive. I also think he killed MCs mother, because as we just learnt he felt MC didn't pay attention to him, but if his mother was dead, especially on the night the king was killing his entire village, SML would be the one MC would run to. He planned the entire thing to get MC for himself, but he didn't expect MC to go to the king instead.