I would think that if the others were killing them, you'd see some sign of injury. Seriously - they don't need the heat lamp. Overheating can kill chicks.
First of all, they don't need the lamp. Mama is there to keep them warm. She has the perfect temperature and humidity for them to hatch in. I don't mess with my broodies until a day or so after the chicks have hatched. I don't know why the babies are dying, but if there is something wrong with...