The true purpose of natural selection

Darwin writes, "Natural Selection cannot possibly produce any modification in any one species exclusively for the good of another species; though throughout nature one species incessantly takes advantage of, and profits by, the structure of another. But natural selection can and does often produce structures for the direct injury of other species, as we see in the fang of the adder". Normally when I think of natural selection, I think of species becoming better equipped to survive, but I continually cease to remember that they become better equipped at the expense of a different species. While natural selection itself may be a peaceful process, its results are quite the contrary. Why do crocodiles have such powerful mouths, to crush their prey. If natural selection did not work the way it does then an overabundance of animals would exist, and eventually the popoulation would reach its peak, and fall quickly due to the shortage of food.