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.
Interesting ...
Funny thing is their letters and diaries are not even necesary ... there is litteraly tons of places where you can find mentions of Moonrise Tower. :-/
Its in the room where Nettie takes you ...
You can find it in priestess Gut room ...
If you talk with Gut, i believe there is chance to get that info ... (not sure here)
I believe even Minthara leave some notes at her war table ...
That halfling Zhentarim vendor will mention it ...
And if im not misstaken, at least one goblin can mention it too ...
Problem is that someone can miss all of them.
It not easy to be honest, but its certainly possible ... and that is i believe the reason they decide to let them hold another diary.
Fun fact ... when you join Goblins and kill Halsin, he will drop his diary ... when you kill Minthara, she also drops Halsin's diary ... it might be bug, but i finded it quite amusing, that big daddy Halsin writed 3 diaries at once, so he can left one in groove, keep another one for himself, and being somehow robbed of third even if he was in bear form whole time.