well, picture says everything i guess. idk how so many spawned without getting killed by the killerjoe. so we really need the moblimit feature.
it was caused by a chicken... because it happened again today, and the killerjoe kept attacking but didn't kill anything. so i spotted that chicken which probably spawned with a zombi, killed it, and killerjoe went on killing all the hostile mobs. so one non hostile mob can block the killerjoe, as it keeps attacking the non hostile mob which doesnt take any damage. solutions would be to contact enderio dev to tell him to cycle through the mobs in front of killerjoe, or enabling non-hostile mob killing for killerjoe on the server.