It determines when an actor dies and checks their health and gives the player an according amount of "XP" based on the amount of health the monster had.
There's no need to redefine every single monster ENTIRELY, as you'll find inside, each monster is redefined with a spawn state which activates the acs scripts, then goes back to the original actor and acts like normal.
Also I included a script that checks when a monster has first taken damage.
Not much else to say. Feel free to use it as you wish, just credit me if you do.

Here's a video demonstration.
And here's a download.