Yea I would get rid of him.thats weird.but if he killed one he could do it again.they will peck to mate but not to the death.unless hen was bleeding then roosters and hens will peck her to death as they are attracted to red and will peck another chicken to death.
I would thank them for allowing you to have all the other pets.and reaffirm them of how well you have taken care of all the other pets.it could be there worried that you have to much on your hands already.
just lost one of my chickens to crape myrtle berries that become the flowers.its the only thing i found on the ground.and i had just cut the grass before i let them free range.and those berrys was not there before.be carefull with these.i believe this chicken flewq up and knocked it off.