This article provides a reasonable explanation of the differences between personal morals and group ethics. That there must be a difference between the two is self evident - societal rules would be unlikely to appeal to all individuals, whose convictions have been shaped by their differing experiences. Nevertheless, normative group behavior must exist for society to function smoothly, and to avoid conflicts between individuals whose personal convictions differ too widely for compromise.
I might add that personal morals and group ethics can both be wrong, and that ideally, they could serve to correct one another. A person with potentially destructive personal convictions could be constrained by the necessity of cooperation with group ethics, and problematic group norms can be corrected by the resistance of those with alternative moral convictions.