What happened to your chicks was tragic and I know how badly you must be feeling about it.
Someone mentioned a predator perhaps being the thing that killed the chicks, and I tend to think that very well could have happened. For the older ones to have killed all six pullets seems rather unlikely to me. That they feasted on the remains would have been natural for them to do, though.
To try to avoid such a horrifying event from recurring, I have two suggestions for you. The first is to go over the coop with an eagle eye and make sure it's completely predator-proof.
The second suggestion is in the future, when you decide to merge a new batch of chicks with your existing flock, after the initial get-to-know period of separation by a fence, cut small pop-holes into the fence so that the youngsters can run back to safety in their haven away from the older bunch. Having a safe place to retreat to, I refer to it as a "panic room", could have spared the severe injuries that could lead to death and the grizzly scene you encountered. By the time they're four or five months old, they should be ready to deal with life in the flock without the panic room. I've had tremendous success merging six-week olds with adult chickens using this panic room strategy.
Also, food should not be doled out by the scoop for "mealtime". Each age group should have their own feeder which must be kept full at all times so that they can eat free choice. If you toss out a scoop-full, the most aggressive will eat it all, and the others may end up starving. The full feeders enable the lowest ranked to get their share of food when the aggressive ones have eaten their fill and are focused on other things.
