Yeah, they were very happy to help the the descendants of the crimson dragon (the fact that they could identify them proved that they weren't scam artists) Him mascaraing the very innocent priests inadvertently cause his wifes mothers sucide. He murdered so many innocent children. Personally, I think his end was fitting, if not lenient for all the horrific things hes done to other human beings.
Priests weren't innocent, but however, yona weren't born as wise as we see now, she isn't blind folded about her position because she didn't knew until every thing happened. She even were selfish and airhead and know nothing about the country in the beginning of the story, because her father though that just because she is the dragon god she will be a good ruler which is not true AT ALL.
Meanwhile, soo woon isn't that bad as a ruler or blindfolded by revenge, he even said that if the king is good he will do nothing!, but in reality the king messed up!
Anyway, I don't know about future, so woon might even die and yona can be the queen.
Well most of the priests died before Yona was born(and they did give people who had 'spiritual' powers a home, the priests weren't perfect but the brother didn't try and spare a single life). I believe the reason why King Il probably became so obsessed with the crimson dragon, was cause he was quite vulnerable mentally due to being constantly compared to his brother. A part of me believes that maybe Kashi foresaw everything. If you remember back to a flashback that soo-won had where he killed King Il. King Il had stated that Yona would be the one to kill Soo-won so maybe every single thing he did from allowing Soo-won to become closer to Yona to letting himself be murdered was due to Kashis premonition. Kashi herself had stated even if you know the future it can't be changed. That would make King Il being adamant that Yona will save the Kingdom make even more sense.
I agree that Soowon's father was someone who could commit horrible crimes both in war and while protecting his wife.
But I'm not entirely sure the priests were innocent, from what I read they might have been trying to get a lot of power by controlling the king and prince Il. (they had that huge ass temple and so on.)
Still no reason to just go and kill them without a proper trial though.

The title states, "The pain that was inflicted on those who inflicted it". It really is a series of domino effect. Let's say Sowoon's father really did not kill Kashi, but who says he is still innocent after he killed those freaking innocent priests. And now King Il killed him, and King Il was killed for that murder through his child. This is such a domino effect and Yona is the sole person in the middle of the things being fought upon.
She is neither paranoid by the crimson dragon king and the belief because she is the sole reincarnation of him, nor is she blinded by power by the fact she knows that the dragons are real. She is the single piece that can end the vengeful cycle that their family began.