It is the job of the DM to let your characters use their skills and abilities to affect the world around them. It's a pretty easy check to fail, even if you are specced for diplomacy, let alone if you are playing anything other than a Warlock. A rogue or a fighter is basically doomed to fail the check with no other alternative. A wizard with charm person *might* think to use the spell before the interaction, but that's unlikely for most people.
Karga is not supposed to be an evil character, but her introduction is to kill a scared child in front of you, then act sanctimonious about it after the fact. That's just bad story telling.
