It depends on how you define "betraying" really. Jowy fights because he wants to end the war. He said so that if one of the Highland or City-State is still there, there will be new wars in the hands of future leaders (tho I think it's just the way it is, you can't really prevent war forever). He wants Highland and City-State stays under one banner so that children like Pilika can live happily without having to feel the burdens of war.
Kiba and Klaus switches side because a) Agares is dead b) if Luca wins, more City-State people will be killed and thus Highland will be known as a ruthless kingdom, ruled by a mad prince who only lusts for blood, Highland will never be the same. Also, I think he feels what the other SoD feels about Riou, that he can actually end this war.
About killing soldiers from your own country, I believe that every person must realize that in war you would have to kill people and thush the act of killing should teach you the meaning of life. So, even if Kiba and Klaus killed so many Highland soldiers, they're killing them because it's necessary to end the war (unlike Luca who kills for fun). Seed and Culgan, Jowy, Riou, everyone fight for this one reason: to end the war. It's just a matter on who do you think have the most power to end the war. I think Kiba and Klaus think that the CIty-State have more chance.
And I don't think Kiba and Klaus would be spared if they go back to Highland. Luca is not a general who knows the value of his subordinates. He kills his own men without thinking about loyalty factor. Kiba and Klaus can be called a traitor, but atleast they fight for what they want and to me it's not betrayal, it's a beautiful thing.
As of Jowy, his "betrayal" to Riou is also what makes him a good man. It's not easy to betray people or things you love but Jowy, Kiba and Klaus did that and they bear the burden too. Jowy could just fight alongside Riou till the end, it would be harder for the Alliance Army to destroy Luca.
But thou art wind!
Do not scream:
thy fragrance deafens.