@raj....well pal Hl2's pirated copy had more than Just the Steams Validation Bypassed......the ripped copy had just made the 6disc Hl2 into 3 disc game by Removing all the Add-On Mods like Hl2 deathmatch and CS:Source.....
Btw i suppose this Bug had been fixed way back also The command is just a lil bit more
ai_disabled 0 (Enables the Ai for that level)
restart (for the level to restart and resume with the Ai enabled)