I'll try to explain. I know it seems a bit barbaric but it's not due to being irresponsible, or heartless, or neglectful.
If we can put aside any moral or ethical discussion for this and look at the realities of incubation and hatching instead it would be helpful.
In the last three days of incubation the eggs need to be set in a particular orientation in order for the chicks to pip and hatch properly. Studies show that and I've had some experience of this here that eggs that are disturbed and left in the incorrect orientation during these last three days are likely to produce deformities, neurological problems and other health issues.
Chicks in eggs communicate with each other and their mother. Science suggest that one of the reasons they can do this is so the hatch is coordinated between the eggs and mother. The ideal hatch is completed in less than 12 hours. This means the mother spens the minimum amount of time virtually defenseless on the nest and all the chicks, being of approximately the same stage in development have and equal chance.
Apparently some chicks have already hatched and instead of mum sitting there quietly making mothering noises to those that have hatched and those that are still in the egg, she's pecking her newborn chicks to death. The distress calls from the chicks she's killing are being heard by the chicks in the eggs and all the chicks that may have hatched. In effect you have chicks being born into a horror movie and not some warm and comforting environment.
Next, while the mother is killing the chicks she's not concentrating on keeping the unhatched eggs in the correct position and the likelihood is the dying hatched chicks are thrashing about among the unhatched eggs causing them to tip and roll. This means that any further hatchings are not going to hatch in the right position and the consequences of this are mentioned above.
Next, how does one assess the health of a newly hatched chick. It may look alright but say blunt trauma to the head or other parts of the body are not going to be apparent until later in the chicks development. You could easily with the best of intentions care for chicks that are already too damaged to function properly in later life.
The there is the problem of care and integration later. Mother is very unlikely to accept these chicks at any stage.
Finally, it is simplest just to assume that the mother hen is unbalanced or a homicidal maniac. It is quite possible that the mother knows there is something wrong with these chicks.