I don't know. How sure are you that she did the killing instead of a rat, weasel, another hen, or something else? One time I had a two week old chick kill a sibling and try to kill another before I intervened. At first I thought it was the broody but nope, it was the sibling.
I had a hen hatch eight chicks and killed two of them before she brought them off the nest. She killed one red and one black, the rest were also reds and blacks. She raised the others fine so color was not the issue. I was out of town when she hatched but the chicken sitter saw her so I know it was her. By the time I got home she was taking good care of the others.
This story is more involved. I put some eggs under a broody at the same time I put some in the incubator just to spread the chances of a hatch. The broody hatched two red chicks a couple of days before the incubator hatched. The incubator was on time but the broody was early, go figure. She brought those two red chicks off the nest before the incubator chicks were ready to give to her. I gave her the incubator chicks at night, the next morning she had accepted the red chicks but had run the black chicks off. I had to raise them myself. She didn't try to kill them, just ran them off. To add to the confusion she had raised a brood earlier that year which contained black and red chicks. I think this time she had imprinted on her red chicks and the black chicks were different.
I don't know why that hen would do that. In her mind there could have been something wrong with those chicks, whether there really was or not. There could have been another reason she selected those two. I don't know how safe the other chicks are with her either. She may be a dedicated mother, they may be in danger. There is a certain amount of unknown that comes with living animals. Chickens are not the only animals that the mother might kill her offspring for who knows what reason.