Make sure that the driver to your 3D graphic card is proper installed and working.
Test if UT2004 is working properly.
cd ~/Desktop sudo unzip AlienSwarm-v13.zip -d /usr/local/games/ut2004
cd ~/Desktop sudo unzip AlienSwarm_13_or_131_to_132_Patch.zip -d /usr/local/games/ut2004
Save alienswarm.png to your Desktop;
cd ~/Desktop sudo mv alienswarm.png /usr/local/games/ut2004/AlienSwarm sudo nano /usr/share/applications/alienswarm.desktop
add the following;
[Desktop Entry]
Name=Alien Swarm
Comment=Alien Swarm is an overhead view tactical squad-based shooter for UT2K4.
Exec=sh /usr/local/games/ut2004/AlienSwarm/Alien-Swarm-Linux
Icon=/usr/local/games/ut2004/AlienSwarm/alienswarm.png
Terminal=false
Type=Application
Categories=Application;Game;
Then save (ctrl)+(o) and exit (ctrl)+(x).
You can now launch Alien Swarm by Applications tab —> Games —> Alien Swarm
Enjoy!