On the Red Letter mission, once you get into the room where you get your suit with Alyx, Barney, the Doc, etc. activate "sv_cheats 1" and then activate "noclip" so that you can walk through walls. Then, after Barney opens the door to your suit, head straight at the suit with the cheat on. You will see open space with a small building up ahead. Head towards the building. When you get there, you will see a Metro Policeman, a civilian, and the G-Man. Activate "impulse 101" to get all of the weapons, and then do what you like. You can kill both the G-Man and the Metro Policeman, though you cannot kill the citizen/bystander. If you want you can type in "noclip" again so that you can walk around. After your finished, just return to Alyx and the rest of them and continue the mission.