"Did Manifest Destiny, Devine Right or whatever other rational is given, really justify the slaughter of peoples, nations and tribes in order to steal their land? And if land is stolen does the weaker party have a right to have their land given back? Does time transfer the rights to the land that was taken from decimated peoples to the decendants of the victors?" Civilization happens. Some Europeans were similarly slaughtered by other superior tribes in ancient times. Notably the original inhabitants of England were conquered and overrun by Anglo-Saxon tribes from Germany-Holland. This has happened throughout history. You cannot just blame European people for this. Every region in the world has experienced this.
Not quite ~ the original inhabitants of Great Britain were overrun by an early wave of Celts who apparently ATE them all ~ every one.
Subsequently various conquests were much less violent.