so really most games by valve actually take place in the same world with intertwining storylines. We all know that (SPILER for those who havent played portal 1, and halflife series)
apeture science is black mesas competiror
and the apeture science lab in L4d2 might actually have some significance since the zombies in l4d2 might be the result of some disease the combine brought over or the (spoiler for thouse who didnt play portal 1)
the neurotoxin released by Glados and supposebly killed everyone, actually turned everyone into zombies. This would explain why there are no dead bodies or bones in apeture science
dont hate people who rip you off,cheat and get away with it, learn from them