I do not like it when NPC just drop a letter when you kill them.
Freedom to play however you want is great, but freedom includes also freedom to fail.
suggestion:
In case you kill both sides there is another way:
In the goblin base is a special room. Its locked and guarded and if you go close to it you will first be warned and then attacked.
In the room there are traps and a locked chest. In the chest are the orders of the absolute for the goblins with a hint of their position.
But it is encrypted so in order to read it you need to kill the goblins (or manage to sneak past them), get the letter and then go somewhere else and do something else to read it.

Prof. Dr. Dr. Mad S. Tist

World leading expert of artificial stupidity.
Because there are too many people who work on artificial intelligence already