Based on the ultimate outcome of Nineteen Eighty-Four, does George Orwell believe that humanity is essentially evil, or just prone to harming itself?
Orwell does not seem to be saying that humanity is essentially evil, but it is prone to harming itself in one important sense. If man is insufficiently vigilant against totalitarianism, he is capable of creating institutions that can destroy all that is human. He claimed, after publishing the book, that part of his motive for basing it in England was to point out that totalitarianism did not necessarily have to remain the fate of people in Russia or other cultures traditionally associated with autocratic governments. It could occur in England as well:
...the English-speaking races are not innately better than anyone else and that totalitarianism, if not FOUGHT against, could triumph anywhere.
Orwell believed that dangerous ideas were taking root in Western society, and that they could be used to justify the expansion of state power to the exclusion of individual rights and human freedoms. Humans were not inherently evil, but even when acting with the best of intentions (as he underscored in Animal Farm) they could create a state apparatus that could expand past the point of human control. By the end of the book, Winston has learned that resistance to Big Brother is futile, and his "love" for Big Brother is a form of surrender to this reality.