ShinAh still has a very naive view of the world. he was isolated growing up, the only people around him being those who feared and hated him. so when confronted with the person who hurt Yona, the one who believed in him and gave him a chance at freedom, he becomes protective. he fixates on the harm SooWon caused and can only view the man as an enemy. ShinAh wants vengeance in a way even Yona doesn’t, in way even Hak at this point doesn’t.
ShinAh sees things as black and white; he only recently began viewing himself in a nuanced light because he’d been raised to believe he was only capable of harm which means he must be Bad. but now he recognizes that he never wanted this ability to harm and can choose how he uses his power.
in ShinAh’s eyes, there is a major difference between him and SooWon - ShinAh never wished to hurt others (unless it’s to protect people) whereas SooWon planned for years to hurt those closest to him (albeit for a higher goal). to ShinAh, Yona represents Goodness and freedom, so those that oppose her must also oppose those things.
i believe even if he did understand why SooWon did what he did, ShinAh still wouldn’t forgive him, because the reality of the situation is that SooWon did hurt Yona despite his fondness for her. and that is something ShinAh can’t comprehend or reconcile. to see SooWon as being in the right would be to accept Yona was in the wrong (even though she herself recognizes her own responsibility as the ignorant princess). but if Yona, the person who saw the good in him, is wrong, then what is ShinAh to make of himself?
ShinAh has not yet reached a point where his belief in himself is secure. his identity is still dependent on Yona’s acknowledgment. it would destroy his newfound worldview to extend any measure of understanding toward SooWon, thus he defaults to wrath.