Resolving cannibalism caused by aggression is pretty tough. It requires studying your flock to figure out what it is that's setting off the aggression. it will require some lengthy observation of their behavior.
Things that contribute to aggression are mixing incompatible breed temperaments...
If this doesn't heal, it's probably a squamous cancer. I had a hen with this a few years ago, and it refused to heal no matter how much I did. It was very painful for the hen. Chickens normally don't show reaction to pain, but she cried when I touched it. It also caused her feathers to fall out...
I confess I have a difficult time trying to make sense of two dimensional photos. Your efforts are appreciated, though.
Since I cannot clearly determine where this lesion is in relation to the vent or if it's actually a vent prolapse, I will advise you to care for this as an infected wound. At...
Instead of all closeup photos, it would be helpful to see a longer view of the chicken so we can place the location of this in relation to the vent. It's called "context".
As it is, it appears to me to be the vent itself that has prolapsed and is infected from pecking damage. If this is not the...