What's the actual difference between these two monters, seeing as they apparently both use the same sprites?

Blood Fiend is quite tough, and vomits blood as a projectile attack.

Mauler Demon is slightly tougher than the Demon, but doesn't inherit from Demon, so it could be problematic. (And overall it seems less interesting as a monster.)

Should one of these be removed?

the mauler demon

Eriance's Blood Fiend spits weaving projectiles, and comes with a spectre-like variant. KDiZD's Mauler Demon A_SkullAttack's as an attack (with absolutely no warning at all, making any KDiZD Youtube run of mine exceedingly frustrating). I don't think it's worth cutting either of them. If anyone has a problem, someone can come up with a sprite edit or something (for the Mauler Demon. Eriance sprited the monster, so his creation should get the originals ).

another 2 items that use the same sprites are SPAS 12 and Pump Action Shotgun

Colored differently, so it's fine. Not liked, but fine.

Mauler and Blood literally have the same exact sprites. Mauler, however, is far more annoying (Whose bright idea was it to give the demon A_SkullAttack?). Blood has "Projectiles".

Both deserve to stay, but Mauler needs a new set.

Indeed. Once someone does that, I'll edit it to have a sort of warning frame before the A_SkullAttack. I might update my local copy of KDiZD to add it, for when I start my youtube run of it.

oops!

i got an idea to solve this. Why not edit so mauler demons have Dark red horns (to simulate the blood dried on them from when they stabbed something with their horns in the charge attack) And make blood fiend drip blood from the mouth and making the skin a slight bit brighter?

Hard to pick out small details like that, in the middle of a game. It'd go a lot better to do some kind of separate edit for the Mauler.

How about a simple palette swap, then? Blood Fiend stays red, Mauler is something like...uh. Black, maybe.