You said "massacre." I don't know that the word allows for a definition that precludes murder.
mas·sa·cre | \ ˈma-si-kər \
Definition of massacre (Entry 1 of 2)
1 : the act or an instance of killing a number of usually helpless or unresisting human beings under circumstances of atrocity or cruelty
Using this definition, hard to imagine a legalized massacre. Even if the US decided to legalize it, international law would kick in.