You want playing Multiplayer but you can't because don't have Serial Key? Don' Worry i'll help you


Firstly, you need create account in https://veniceunleashed.net/signup


After create your account. Next you need to download Project Rome Files[veniceunleashed.net]
And extract it, then move the .dll to your BFBC2 Folder (steamapps/common/Battlefield Bad Company 2)

After that, you just start BFBC2, Login with Venice Unleashed Account, and Enjoy

Sorry if my English bad :)

Source:
https://www.pcgamingwiki.com/wiki/Battlefield:_Bad_Company_2