"To the victors go the spoils".  Pretty much every culture on the planet has done this.  

The Blackfeet used to raid the Shenone, kill the men, and take the women as slaves.  The Iroquois were notorious for this practice too.  

There is a difference between "robbing" and "conquering".   Robbers take and leave; conquerors take and stay.  North American aboriginals were conquered.  The body count was much less than it would have been otherwise, because they realized they were out-numbered and out-gunned.   The signing of treaties avoided a ton of bloodshed, even if they have been implemented imperfectly.