Well, it wasn’t just plunder. They cut off their scrotums. Disemboweled the dead. All other kinds of acts. While it may have ben customary for some, it wasn’t customary for the times. Arguments can be made for atrocities of course all around in those times, however it was considered especially heinous even in those times. But I understand what you are saying.
For your information, scalping was done much earlier, Indians were scalping enemies centuries before Colombus came. Mutilating bodies was Indian customs and Indian religion since the beginning.