You play devil's advocate because you want to test your convictions by making a good faith effort to argue for the other side. You attempt to present the other side's argument in the best light, and really think about your reasons for actually concluding against that side.
Trolling is the opposite of a good faith effort; you are intentionally making an argument you know is absurd, in an attempt to get someone to engage as if it were a real argument. You have no intention of trying to understand the other side or think about why people might feel differently, you are simply trying to confuse and anger people.
You can argue that you think trolling is funny, but you can't argue that you posting inflammatory things is playing devil's advocate.