Confirm it's pirated version.Since when L4D comes with a setup and TS have no clue about Steam.
TS should know that he should never ever pirate a Valve game.Pirated Valve games usually give a lot of problems.Crashes,AI issue,unstable etc.
If it's original,just go to Steam,right click L4D and delete local content.Thats it.Simple.
Left 4 dead unable to Uninstall