Most times chickens peck each other to death when they are extremely disturbed.
When you remove a chicken or add a new chicken to a flock of chickens the other chickens can hate that and not like the new chicken in their flock.
So the chicken tries to harm or kill the chicken by pecking it to death because it does not like it.
Not all chickens will peck new chickens to death but some will.
If you have new chickens or chickens that are pecking each other then you may need to keep them separate at least until they get used to each other.
Some chickens just do not get along with each other at all and need to stay separated.