: Setting the player / marine to "enemy" in a monster's physics
: settings should be enough to make it necessary to kill that monster, yes?
No, that's backwards.
The player has to kill every monster set to "enemy" in HIS OWN physics. If a monster has you marked as his enemy, he will try to kill you, but unless he is marked as your enemy, you don't have to kill him. You could have monsters that completely ignore you and yet you still have to kill them.
I believe, when 'rescue' actually worked (in M1), that the "friends" list in the player physics did a similar thing, i.e. you have to rescue all the 'friends' and kill all the 'enemies'.