Um, what. The corruption of Chaos is precisely behaving bad - first, it is cutting someone a little, then a bit more, then a week after that you begin rambling about skulls and buy a Chainaxe to do a little bloodletting. Chaos is subtle, it doesn't corrupt anyone by dealings or contact, well, unless they're Tzeentch or Slaanesh, and even in these cases, your behavior is perfectly sufficient.
The part you mentioned, books and dealings, is used only when you need a quick fix of power, or some kind of deal. If behavior slowly heats the proverbial water until it boils, the deal hits it with lance battery of corruption.
You know, I think what you did was... well, pointless. If I wanted to keep him from escaping, I'd break his legs or spine, if I wanted him talking, I'd bring the real torture (though, that's pointless, too, unless you can check if they say the truth, as they will lie). This is kind of both, kind of useless, and it might be well a corrupting behavior of someone more sadistic than he needs to. I'd need more data to decide.
As for the party, yeah, if he won't talk, and if I can't keep a prisoner, why bother? Only if you know he knows something (which is unlikely in a grunt's case), and you can walk him out, I'd consider leaving him alive.