Larian is DM. DM decides what is good and evil. Goblins are painted as bad. Tieflings good. Join goblins. You're bad. Join Tieflings, you're good.

Therefore, based on DM's choice, kill all goblins. Good. Kill all Tieflings. Bad.

Just ask Gale, Wyll and Shadowheart. They all don't like helping the goblins.