Can't play Arkham City

I was able to launch Arkham City a few days ago with no problems, but when I tried just now I am asked for a serial key which I have only been asked for in the past after installing the game. In the past I just put the cd key in, but now I am being told that is wrong. Now I know the GFWL has been removed and in my steam library there is also Arkham City GOTY edition which I never bought. Do I have to install the GOTY to play now or use the GOTY cd key on my standard edition?

Probably one of the reasons they gave people the GOTY edition, the standard one probably doesnt work without GFWL or something. I just reinstalled the game using the GoTY edition and works fine, not sure about standard edition. In any case, it's an obvious choice to play the GOTY edition since it contains the DLC as well.

Yes, you need to use the GOTY edition. Be aware though that this transition does not carry over your saves.

That sucks I lost my saves, but at least I can finally play Harley Quinns Revenge. This will probably sound stupid, but there is no reason to keep the standard edition of Arkham City installed now right?

Yeah. No reason to keep the standard edition installed. In fact, it might just merge itself with the GOTY edition once you uninstall it. I had the regular edition of Arkham City on steam already, with GFWL, but it wasn't currently installed when it got updated to remove GFWL. In my steam library, it just changed to the GOTY edition, so I don't have both separate.