If the lamp is lit, monster deactivates ?

[FlawlessHappiness]

I suppose this is only going to happen once?
That sounds pretty easy. What you need is a function for when the monster spawns (Which you already has) and a function for despawning it.

Good. So spawn the monster and unlit the candle when you want to in the script.
Then, create a callback function called: SetEntityCallbackFunc.
This function is pretty cool.

Good so in your script you make it like this:

void CALLBACKFUNC(string &in asEntity, string &in asType)
{
if(asType == OnIgnite)
{
//Despawn monster
}
}