The biggest problems that man causes occur when he starts thinking that he is somehow above or not part of nature. That has never been true, and never will be. Man is a member of the animal kingdom, and until that changes, man will be part of nature.
It might be a good idea to wonder in what ways man is different than other animals. The differences are profound, but are also part of nature.
Man has an opposable thumb. This gives him the ability to use tools that he would likely be unable to use without this trait. This does not, however, make mankind less a part of nature. For one thing, chimpanzees and other apes have the same capabilities, and many of them do use tools, including the aforementioned chimps.
Man also has a highly developed brain. However, earlier species of man had a larger and apparently more developed brain, yet they died out. Also, brain development doesn't necessitate intelligence. For that matter, studies have shown that a porpoise is at least as intelligent as a man. Few would dispute that a porpoise is part of nature, so intelligence and brain development and size doesn't elevate a creature above nature.
How about the works of man? Man can and has shaped and changed his environment. He has taken over what he didn't like or couldn't understand. Doesn't that make him above or outside of nature?
It might, if no other creature did this. But at least two do: Rats and viruses. In both cases, the creatures shape and change their environments, and in both cases, like man, they will occasionally change their surroundings in ways that will prevent them from living there. In fact, viruses live off their host in much the same way that man lives off the earth and everything on it.
Perhaps a better question to ask is why human beings don't realize that they are part of nature, though the answer is pretty much the same. It comes down to simple arrogance.
Mankind thinks that he is so powerful that he is changing the climates all over the globe; though there is no proof at all that he is doing this. He isn't concerned, though, with the great amount of pollution he is spewing into the environment. He has the unique viewpoint that he can destroy what he wants, when he wants, though he is also capable of such outstanding beauty and doesn't seem to give that a great deal more thought.
Are human beings and mankind still part of nature? They have never stopped being part of nature, and are very unable to do so, regardless of how much arrogance and egotism they can muster. For good or bad, all animals act in accordance to their nature, and unfortunately, the same is true for mankind.
The biggest key is the question, will mankind ever realize that he is part of nature? The answer to that one is still being determined.