We had seven chickens. Three nights ago a hybrid was killed by a predator. Two days later our other hybrid's comb was destroyed. Now the 5 remaining chickens are pecking the damaged hybrid relentlessly and brutally. Did the other chickens do the original damage to the hybrids comb or, was it a predator and the chickens are trying to finish the job? We have never seen this kind of behavior from our flock.
Actually, that is normal behavior. Chickens will pick at a wound relentlessly till they kill another bird-they can't help themselves. I have my flock rooster separated now because he got a couple small wounds on his wattle and comb from fussing with another rooster through the fence. His own girls picked and picked till they got a chunk out of his wattle and he was bleeding profusely. He could actually have bled to death if I hadn't caught it. Even when it was about healed and barely noticable, I put him back in with them and the same girls were looking at the former wound closely and within two days had it picked open again. Nothing would deter them, at least so far I haven't found what would. It is just in their nature to be cannibalistic. It was probably the predator who caused the original wound and they are now focused on it. You'll need to separate the wounded bird till it heals or they will continue. Even Mentholatum/Vicks Vaporub applied to his wattle didn't stop them from picking at my poor rooster. Yuk.