I've "killed" Monster many times in the original game.
Waterlurker: Pick up a Barrel, wait until the Lurker comes close, then slide the barrel against the floor and right click to throw it. When it slides towards the Lurker and makes contact, it will make the Lurker fall through the map, and "die". http://www.youtube.com/watch?v=L1i4ZD06GgE
Grunt: Tricky, but doable. Pick up a BOX. Wait until the Grunt comes close. Right click to throw it, and make the grunt STUNNED. While in it's stunned animation, lightning fast, pick up a Box and throw it at the grunt. The Grunt will glitch, and it will be stuck in either it's walking animation, or a super fast running animation and it's "killed". But you will still suffer Sanity damage if you look at it.
Brute: Next to impossible. Go where there are alot of objects. When the Brute appears and walks towards you, pick up a box and throw it at the Brute. When it's stunned do the same as the grunt and Throw the box at it, but it won't be glitched, next do it again and again about 6 times, and it's done, it will either fall through the map or it will be stuck.
It is possible to "kill" them. But it's hard, well except the Waterlurker, that one is very easy.
For the Brute, you WILL DIE the first time you try it... YOU WILL DIE.
The Grunt, throw throw, done.
Peace out! Amnesia Completed killing the monsters. I never had to hide even once.