Office politics - how to deal with them? what to do when?

You already answered your question: The GUY is leaving and probably has the biggest chip on his shoulder - stay away from him. Don't share any of your opinions with this guy - maybe you are the one who is replacing him and you don't even know it. Never and I mean NEVER go to lunch with this guy - he is leaving soon and there is no reason too. Do you want him to be able to start some type of rumor before he leaves about you? Bottom line: Talk very limited too him until he leaves.