1) After killing Sharron, we are no longer needed by the Fog People.
2) Objectively, we are unlikely to be able to take revenge on them with the remnants of our squad (even at full strength, it would be unlikely).
3) During the unrest in the city, all the valuable Fog People should quietly leave – there will be nothing for them to do there anyway. Should they deliberately stay to deliver the medicine? What's the point?