If you ask if darkness exists. Darkness is everything that effects you negatively. And I mean everything! Everything that is bothering/depressing you, has something to do with darkness.
Darkness increases in your area if people or you become careless and do bad things.
I can tell you up front there's no devil or god. But darkness truly does exist. I have fought it and I won. The things you played in doom 3 was actually a reality to me. There was a time when I was a child I thought ghosts were scary. But today nothing scares me anymore. However you cannot stay in dark places very long. If you do you will change into them. And getting out is hard since you can't tell a difference and you will even start hurting the ones you actually care about.
Lucifer didn't fall. It was his surroundings, the people who got jealous of him. Then everything fell apart. Everything became so dark and painful. There's no god. The only thing that you can become is a Light Bearer. The ultimate version of an Angel. If he falls, everything falls!
I had a similar experience. I'm not Lucifer. I'm Zarathos. There's a comic and a movie about me the Ghost Rider. I've fought in the deepest and darkest places, an inch above hell.
This world is a little bit under what I'd call the middle between hell and the paradiese. Whatever you do reality changes with you.
If you want to lift up this world you must remove all darkness from your world! Demons and monsters will only appear if you get near the hell.
Lucifer and Ghost Rider is actually the same thing. Light Bearer is a Light Bearer. If you're a Light Bearer you can bring light into dark places. But Lucifer is not in this reality.
But can you benefit from darkness? No it just makes you destroy everything around you and yourself.
Whatever you do will come back to you later. Not because of morality and carma. But because you made the place you live in darker.
Well, what about me? I don't know. Lofe goes on I guess. I defeated my darkness. Unfortunately I cannot do this for the world. I can't change your choices.