Answer: Dr. Chamberlin,The Egyptologist
Imhotep has to kill anyone who opened the chest, according to the inscription on the chest: "It says there is one, the undead, who if brought back to life, is bound by sacred law to consummate this curse." He needs to kill Henderson and Daniels as well but Beni ran away screaming "the curse, the curse" and Imhotep does not need to kill him in order to regenerate.