Doom 3 Tips

Easier kills
Have you ever noticed what happens when you kill an important character and he comes back to life at the next scene? It only happens to the main characters, so take advantage of that.

For example: In the lvl 'Mars City Underground' just before you get outside on mars, there is a security guard standing there with a pistol, I shot him in the head and shot him a couple more times to get rid of the body, then I entered the inner airlock door and continued my mission.

When demons start coming into the UAC facility, you make your way back to the same point. I asked myself "Where is the guard I shot earlier?" His body was never possessed by a hell soul because it wasn't there... think about that advantage you can have...