actually unless humans are special (set apart from nature), it would just be another environmental condition. just as much a part of natural selection as if a lion selects for the fastest to breed by eliminating the slow.
You can arbitrarly draw a line between artificial and natural, but Darwin's line was simple and objective. Selection involving human intervention and selection not involving human intervention.
Human intervention was not Darwin's or Galton's invention. Selective breeding was not Darwin's of Galton's invention. Blood purity was not something dreamed up by science. It is as least as old as the geneologies in the Bible, including the ancestry of Jesus -- from royalty.
Killing Jews was not something dreamed up by Darwin or Galton. Darwin was not the inventor of the German passion play, which stirred up hatred of jews. I've never even seen a reference to Gypsies by Darwin.
Scientists seldom aspire to political power. Not many have ever held high public office or even led a major corporation.
I suspect more clergymen than biology teachers have been convicted of diddling children. I suspect organized religion has been responsible for more genocides than have biology teachers. Certainly the ones in the Bible seem to have been inspired by religion.
And in addition to all this, you have the rather mundane fact that Hitler didn't rise to power on a platform of science, nor did he print "Darwin is With Us" on belt buckles. Rather he stood with clergymen on platforms decorated with crosses.
Or natural selection *for* cooperation between members of a population where those most cooperative have more offspring than those not so cooperative.
Natural selection does not select exclusively for the most brutal. In many cases, brutality will be selected *against*, or selection fluctuates between cooperation and brutality over time.