This game has a good concept, and making an FPS in Pico-8 must be quite a feat (major kudos on that). And I like the editor feature! I can certainly tell that you're trying to work within the limitations of Pico-8, and I respect that a lot.
I actually never heard of Devil Daggers until I saw this fan game. And as for the difficulty, from what I've heard, the difficulty is part of the appeal of the original Devil Daggers game.