It's the natural process of evolution. Nothing can evolve without death, or at least some sort of iterative process in which newer generations replace older inferior generations.

The Nazi's unfortunately took this theory to radical proportions and used it to justify the mass-slaughter of those they considered weaker than themselves. In terms of survival of the fittest, the ideology actually makes sense, especially in the context of winning a war, but not in the context of progression towards peace and prosperity. Hitler took the process of evolution into his own hands and "cleansed" the world of all those he considered weak, and on many levels it did help him achieve his goal, in the sense that he successfully conquered most of Europe.

It's one of humanities greatest faults. Love, kindness, and compassion, are all too often mistaken for weakness. Men like Hitler were unable to separate themselves from their violent ancestors and believed that man should be just as ruthless as nature. I hope Trump steers well away from this destructive idea of progression, but I'm not holding my breath.