An aspiring Source Engine modder named Saul has figured out a way to unlock Versus mode in the L4D2 demo, allowing players to play as the Special Infected.
Warning: I have personally not tried out this hack yet. GameSync.net is not responsible for any damage to your computer or Steam account. Use this at your own risk.
No extra downloading needed if you join a game in Versus Mode, just a simple command you type into console. If you want to host a versus game however, you will need to download a simple 80kb file and use a few commands to get you started
First, download the plugin
1. Extract the ZIP to Steamsteamappscommonleft 4 dead 2 demoleft4dead2
2. Start up Left 4 Dead 2 Demo
3. Get a game going in Single Player or Online.
4. Type the following into console:
plugin_cvar sb_all_bot_team 1
5. To become Infected, type the following into console:
6. To change back to survivor, type the following:
7. You can force a client in your server to be infected/survivor by using:
plugin_set_infected <their userid> <0 = make survivor, 1 = make infected>
8. Don’t want your clients to be able to make themselves infected and manually spawn? Then use the following command:
9. If you want your clients to spawn themselves by using infected_spawn, and to disable auto-respawning, use this command:
Instructions taken from the Left 4 Dead 2 PC Steam group. You can read the original announcement from their group page here.