Tragedy in the Coop

I have 3 different age groups in my coop; Laying hens, "teens- 4months", and newly hatched. All were getting along fine until the latest clutch started to hatch. As I was enjoying the miracle of the hatch, I hadn't put my guard up and segregated the nest from the rest of the flock. The teens seemed interested in the hatchlings but were not hostile. As I left to finish placing the top on the cage and kick all the others out of the nesting area, one of the teens snatched a fresh, still wet peep from under the hen and ran away with it! WHY??? I retrieved the peep but it died later that day from it's injuries. WHY??? Why would the teen do this and why would the mother hen let her new peep be snatched out from underneath her? I am soooo devastated! I know this is 'life in the coop' but I'm trying to understand the behavior, so that I might prevent it with the next batch.

Long story short, how long should I keep moms and peeps separated from the rest of the flock? They are currently in side by side coops. (And the rest of the story goes; the teens are not happy that their mama dumped them for this new set of peeps. She shared the nest with the real broody mama. Two hens sharing the 3 peeps now.) It's confusing, I know!