Out of nowhere, group of Elf, Githyanki (wich you only see as some weird shit you never seen before, but you may remember that your another friend, Zoru, inside the camp described to you something simmilar as murderers of another friend of yours), Half-Elf wearing armor full of Shar symbols, and Goblin appears on the hill.
Do you really tell me that you would presume they are friendly?
If that group attacked the horde of goblins that were attacking us, killed them all, and then stopped short of attacking our fighters like what happens in-game? Yah, I'd be willing to give them the benefit of the doubt.
Besides, of that group you described I'd be a LOT more concerned about the Githyanki and the Shar Priestess than I would be a lone goblin. A single goblin is rarely a threat. A single Shar cleric can be quite dangerous. Yet nobody raises a brow about Shadowheart when they see her.
Goblins are only threatening if there's a lot of them. I don't see much of a reason for killing any lone goblin on sight except for the sake of being cruel.
