My flock are coming along nicely, with the first batch laying just over 120 eggs a day, and my second batch growing well, despite losing several to illness despite a constant feeding of antibiotics and vitamins. Recently they've been pecking at hens who have just laid their daily egg, until sometimes they kill the poor chicken. How can I stop this happening?
looks like they need maybe some forage area..something for them to do is very important..if its just dirt they have nothiing to do...
They are probably frustrated from lack of stimulation in the run. Do they eat any treats other than layer mash? Are they allowed outside of the run? If so, they may just be having pecking order issues, but if not, it's more than likely due to boredom and I'd recommend letting them roam for a few hours daily. If you can't, you could give them weeds/scraps/treats and toys, and more hiding places for those being bullied to escape.
Increase protein level of diet. Mix in a little chick grower and make available whatever calcium source you have available. Based on image you are pushing them pretty hard and when in large groups sub-optimal diets can easily lead to unpleasant problems like cannibalism very quickly.