What people don't understand is that Swain and Leblanc, while they hint at a once romantic relationship, aren't going to be betrayed because they don't TRUST EACH OTHER! Leblanc knows Swain plans several steps ahead, and will quickly adapt to a situation. Swain knows Leblanc has a massive network of spies, and is just as hungry to have power, as he.
The Deal is quite obvious. Instead of having someone to manipulate, like the Black Rose once did, they're going to plant their own into the Noxian High Command. Darius has already cleaved some heads for the sake of it, so here, we have Swain and Leblanc. Mind you, I don't really deem them "Villains".
Swain and Leblanc know what Noxus USED to be. They see Darkwill as the man that ruined their home, and their plan is to bring it back to former glory. What more, if Darkwill HAD been doing a good job, he'd been the Grand General for far too long. A sort of "Stagnancy breeds death" thing.
Now, Swain is a patriot. Through and through. Everything he's done, it has been to put him in power, but think about it. What evil thing has he done that didn't have a purpose?
"But StarkravenMad, he's a Noxian, and fought the Ionians fo-" Stop right there. Heavy implications state that Swain disapproved of the war, and was fired because of it. Maybe as a tactical flaw in the initiation, maybe because he believed resources were better off elsewhere.
"He attempted to kill Jarvan!" Yes. He did. I think that was more of a message though. Swain broke a HUGE rule in the league, and what happened? Vessaria plucked him away, smacked his hand, said "Bad Boy" then carried him off, letting Jarvan bleed out a bit more. He was trying to let Jarvan know that he could get him at any point, and that the League wouldn't provide a horrid punishment if he succeeded.
Now, for Leblanc, I admit, I don't have near as much info, but I'll give an example.
A man is on his farm, working the land. Suddenly, the government comes, stabs a flag in it, and says "Mine". Now, this government is big, so you'll lose in a head to head fight...so what do you do? That's right. You plan. You plan and you plan and you plan. You may have to bring in your neighbor, that had his farm stolen too, and between the two of you...you plan.
You're gonna get your crops, dammit.