Could it have been some information from Yu-Hon that made him feel that war was the only option then since the hatred was too strong? Even during Kin province, he was fully determined to get it back because it belonged to Yu-Hon before Il gave it away. Soo-Won is rational for the most part but when it comes to his dad he is a little bit more emotional I feel.
But it is odd that he didn't find out more about Tao and Gobi. Maybe initially he wasn't even bothered since Kouka had the upper hand and would have one anyway? I don't know. Perhaps calculating all the options - war seemed to be the most efficient - since there could have been internal conflicts within Xing later.
Right now though it would make sense to at least talk to Xing. It could very well not work out and war could still break out.
Update: Did you guys read 134.6? It's so cute! Especially Kusa's afterword.
Yeah this is the little reason I can think of to explain Soo-won's earlier actions. He may be letting his emotions cloud his judgement on this one. Just a little anyway.
However, never mind, I just recalled that Min-soo said that Su-won knows about Kouren’s hatred and that she wants war, so maybe he does have another source of information other than Ogi?
This would also explain it a little. If Soo-won knew how much Kouren, the next in line, loathed Kouka, then he would know it was because of his father's excessively brutal massacre in Xing 17 years ago. He may have quickly decided on war thinking talking with Kouren is a lost cause because of her deep hatred. Soo-won may be unconsciously jumping to rash decisions instead of calm calculation, because he knows if he has to think about Kouka's relation with Xing, he would have to contemplate what his father did in detail. He may not want to question those moral and political decisions as to not tarnish the boyish, hero-image he may have had of his father. And when Yona confronts him later about it he may have woken a little and realized what he was doing. Only then does he give in and decide to consider Yona's proposal on peaceful negotiations.
I also think Soo-won may be getting a rise out of Yona always sticking her nose in his politics. When she and Hak come to seek audience, he turns her down. One reason explained above, and another because this was right after hearing rumors of Yona's growing influence and witnessing it first hand in the previous arcs. This was not long after the Sei mess, and there she is again. This time at his doorstep, one step ahead of him on the current issue at hand like always. First Awa, then the Fire Tribe mutiny, the Iza seeds, the Nadai incident, then Sei. She always seems to to be in it before him, sometimes even solving the issue completely. I think one side of him appreciates her work as a childhood friend, but another may be slightly annoyed. Kye-shuk whispering in his ear about it probably doesn't help. That's just my take anyway.
I think Soo-won above all else want to stay rational and not let emotions cloud his judgement on what's best for his country. So if he sees the negative consequences of his father's brutality and the rational political proposals of Yona, despite all reservations, he will try his best to adhere to rational thinking. He will try his best, but let's see how far he can go.
Update: Did you guys read 134.6? It's so cute! Especially Kusa's afterword.
I did a while back on a tumblr translation so it was nothing new to me really. But chapters with the HHB just interacting with each other always warm my heart. We always learn interesting new tidbits about the characters despite the 'filler' vibe you might get from these chapters.
I think it's cute how the Hakuryuu villagers might be covertly stalking their beloved Lord Hakuryuu.
One thing that was mentioned a while ago here of a possible 'return to Sei' arc before the Kai arc. I was trying to remember what actually happened with Nadai. IIRC Ogi said the merchants have disappeared. So if the quarrel with Xing lasts for a while, I'd expect Nadai to resurface once more.
I'd like this to happen. Of course it all has to do with the selfish part of me not wanting this manga to end anytime soon. Due to the limited world building, it may be that conflict with Kai is the final arc of the manga. Unless Kusanagi thinks of other creative arcs without it feeling forced or shoehorned in.