I know most of you find this difficult to believe, but YES! Some monsters actually can kill you!
Some monsters actually have devastating attacks such as disintegrate, polar ray, light beams, holds, etc. That can actually kill players who are not careful.
Some of them have saves.. but some of these saves are super high on some settings.
Some do not have saves.
These kinds of things were added to the game to create a challenge.
Some of these challenges are difficult to counter by using the standard tactic of charging in with a great axe and pushing the attack button.
Some monsters are also immune or resistant to certain spells.
This means that maybe a single spell cannot kill every monster in the game.
These kinds of things are not bugs.
They are there to make the game less boring.