It is one of the mechanisms of evolution, usually expressed by the cliche "survival of the fittest" (except that you define the "fittest"
as the one who survives, so that you have a nice circular argument).
To get a bit closer to home . . . We don't know exactly what killed off the australopithecines after their having lived in eastern and southern Africa for two million years, but it may well be that genus Homo, wittingly or unwittingly, helped.
And Homo erectus may have been done in, at least to some extent, by Homo sapiens, while the Neanderthal variety of the latter was in turn done in by the modern variety.
We can't put ourselves into the minds of Homo erectus or Australopithecus africanus, let alone into what might pass as the mind of trannosaurus rex, but we know very well what our own minds are like. We have minds that make it possible for us to know what we are doing when we cal ously mistreat others who are very much like ourselves, and do you know what we do? We rationalize our cruelty, and justify ourselves, and even make ourselves sound moral and noble.
Here is the first example I know of. Immediately after the Flood (according to the Bible) Noah planted a vineyard, made wine, drank it, and was drunken. And his youngest son, Ham, the father of Canaan, didn't show the old man the proper respect. (The Bible doesn't go into detail.) Noah therefore said, "Cursed be Canaan; a servant of servants shall he be unto his brethren." (Genesis 9:25.) In the time of King David and King Solomon, the Israelites control ed al of Canaan and enslaved the Canaanites and put them to forced labor, it was not because the Israelites were a master race and did as master races always do. Not at all. They did it (they said) because of a Biblical curse on Canaan. (One that was undoubtedly inserted into the Bible after the fact.)