Hey there, I would like to make it so that when the player(s) leaves the zones (goes outside of the map) they get killed, like the way it is in black ops 3.

Sorry for the question, but for what do you need this? You can use clips to keep the players inside the playable area. Or do you want to kill them when they fall somewhere down?

I want this for when they fall down somewhere.

Not sure of this, don't remember very precisely.Make a trigger_hurt and give the Targetname "dmg" and give the value 100 or more(without jugg,if your map  have it give something more than 250)