Is using a CD key found online illegal when you already own a game?
I'm not a lawyer, so don't rely on my opinion (!), but I don't see how it can be. A CD key is a device employed by the publisher to prevent piracy - it has no basis in law by itself.
When you purchase the game, you are in essence buying a license to use that product. The theft of piracy occurs when you use a program without a license. You already have a license (you bought the product), so how you get the program to run is your own business. The only problem is if use of an online key invalidates your license. Have a look at the license agreement when you install the game - if you haven't done anything to invalidate your license you should be OK. And morally you're in the clear (in my book).
However, where a game uses an activation code that can only be used once, that is a different matter. Using an online key would prevent another person from using their copy of the game, which of course would in essence be theft.