Can chickens raise their own broods? Absolutely, and do a fabulous job. So good in fact, that I stopped heat lamp raising after burning a coop down and went solely with hen raising.
But not all hens make good mothers.
However, before assuming your hen killed the chicks herself, I have a question to ask? Is this a communal nest that she used to hatch the chicks. The only time I've had good hatchings to find dead chicks under a broody hen was when the hen was in a communal nest. I inadvertently watched one day as another hen came into the nest to lay and saw momma trying to hunker down, refusing to budge off her chicks. The result is dead chicks as the weight of both hens came bearing down upon them either suffocating or crushing them.
Another option with larger hens is that simply getting in and out of the nest they kill chicks by stepping on them.
Look at the bodies (if you can still). Are they simply dead with no visible trauma? Or is there some sort of trauma visible...a broken rib, squished appearance, bloody head? If a hen is killing her own chicks, she may throw them against something and you find the chicks flung away. Or, they peck their heads.
Also consider genetic problems or bacterial. Do the chicks have swollen legs and abdomens? If somehow the nest was especially soiled, they can contract bacterial infections in their abdomens freshly after hatch, especially if their navals were not fully closed. You will see a bit more of that under a broody where conditions are distinctly not sanitary. However, I've done a lot of broody hatches, and I've only had a couple of omphalitis cases, and both were in rare breeds with open navals, so I suspicion genetic weakneess or malformation as well.
And finally, if they simply were dead, even huddled together in the middle of the nest, and you have cool weather, mom may have gotten up and forgotten to keep them warm. If it wasn't clear where mom went, or not easy for the chicks to follow her out of the nest box, the chicks could conceivably try to huddle together and have succumbed to chilling.
I'm sorry to hear your hen has had such a poor result with her hatchlings, but it will take some investigation to determine if it was a bad mother, bad mothering, or simply a bad location.
LofMc
