How intelligent are various monsters?

[stochastic]

Not sure about their intelligence, but they sure do have a knack for colorful interior decorating:

[Merry Widow]

I'm not sure how smart the monsters are either, but when I try to play Doom while half asleep, they sure seem like they are. Until I've had my coffee that is.....:D

[GeorgeCostanza94]

I've always imagined all of the monsters like wild animals who evolved to be much more violent and sadistic than humans did. While humans evolved to use reason and logic, we lost our physical power. And while the demons evolved to rely on straight up brute force and blood shed, they greatly lack in intelligence, including strategy and elegance in combat. That's just kind of how I imagined it.