You cant compare the flood and the head crabs seeing as the head crabs in hl2 are pretty much dumb. they walk slow and are very easy to kill…..kinda boring. the flood in halo are semi smart and jump around and actually think compared to the head crabs. i dont even know what you talking about with zombies…….
Also hl2 is more physics based and is cool but not good for a multiplayer game. just kinda fun…then gets old. Also you cant compare a game that is mainly pc to one that is mainly console. i know both are on pc and console but halo was originally console and hl was mainly pc. Thats like comparing CS to Halo. They are different types of FPS.
Anyway…..a 50 before the update would have to of been from cheating. A 50 before the reset would have taken hundreds of games and no one has reached it legitly. If i were you i wouldnt talk about getting a 50 when it wasnt possible before the update and is almost impossible now. You dont know the true joy of Halo 2 when all you did was play for a few months,cheat to get your level and quit playing. MLG is the main reason why halo 2 is so popular and it is on the USA Channel on TV. HL2 cant boast that.