As well, I think it's a bit of a complicated scenario for Jowy all the way through. Luca Blight is clearly the 'big bad', right? Or more like, Highland as it was under Agares / Luca. Jowy is compicit in killing Agares as he knows he needs to get close enough to Luca and if someone doesn't stop Luca, he will obliterate Highland and Jowston / Dunan both in his quest for oblivion.
He sets up the rather complicated scheme to get Luca in place to have him executed but what then? Highland still exists. There are still tensions and political meanderings. Jowston isn't united under Riou's flag yet; even if Highland strikes a treaty, the damage is still done and without places like Tinto on Dunan's side, even if a ceasefire / truce is reached, it is only a matter of time before the dissidents cause another war.
For Jowy, the Shield / Sword imagery is more than just something he rants about; he knows (or falsely presumes, rather) as long as he and Riou have each side of the Rune, they will fight. He knows as long as Dunan is fractured and Highland has strength, the wars will continue. So he continues to develop Highland and continues to fight the war. Either Highland will eradicate a weakened Dunan and the war ends. Or Dunan will - in Jowy's case, hopefully - come together, regain its strength and finally do away with the Highland corruption.
When Jowy confronts Riou on the mountain, he doesn't do so as a defeated foe, per se, but he's almost suicidal. You get the feeling he's never wanted to win at all - Nanami's 'death' has brought this bubbling to the surface. He hasn't really been playing to win, and that's really the point.
He 'betrays' you to give you the best fighting chance at defeating the Blight family. He carries on the war because he doesn't think if it ends at that point it will stay ended. He wants Dunan and Highland to be united and to stop the deaths on both sides, but he feels unless he really risks it all, that can't be achieved.
That's how I understand Jowy's betrayal. It makes a change from psychotic 'villain of the week' I have to say.
who still wishes Luca had survived longer, of course, but it was a surprise - and a good one - when he didn't.
"Everything is something, but nothing is everything."