You obviously haven't come across TF2 or any other Source multiplayer game before.
In order to play TF2 you must be connected to the Steam servers with your Steam account, which means you will need a legal copy of the game. It is not possible to just "torrent" the game, because TF2 does not have an installer as such, instead all of it's files are downloaded through Steam, or transferred straight off the DVD. And even if you did run the torrented version of the game, there would be no way to connect to the Steam servers to actually play the game online (no offline modes). The only way to do it would be to employ some roundabout hacks and use a VPN such as Hamachi to play with one or two friends who were also happy to put up with all this palaver and fail just to play TF2 by themselves, and save a few measly $$$.