Hmm, but here's the problem. Nobody ever thinks they're doing bad. We're all the heroes of our own story.
The problem is that people are easily manipulated, and prone to irrational fear. This is when our animistic side manifests. It happened during the French Revolution, when desperate, vengeful people fought for liberty, only to give a handful of people total power. They then killed thousands while searching for counter revolutionaries. It happened during the Rape of Nanking. This happened again in communist China, when people sought counter revolutionaries and perceived enemies of Mao Zedong. It happened during the witch hunts in Europe. It happened in America during WW2, and during the period of McCarthyism.
People aren't inherently bad, but we're gullible, and afraid, and we're followers, which means that evil men can get in power and convince us to support them, usually by creating a foreign threat based on intolerance. To some degree it's still true today, in the way we view the Middle East. I don't think people are generally good or bad, I think they're generally sheep. Just look at the Milgrim Experiment.
That said, and this is very important, to claim that people are fundamentally bad is intellectually dishonest. While that violent, ignorant, side of us is always there, the above examples were rare anomalies. We live in the most peaceful time in world history. World violence has been steadily decreasing for generations. War atrocities have been decreasing. Crime rates have been decreasing, completely independent of the state of our economy. Medicine is better than its ever been. Increased crop yields and a stabilizing world population means that hunger will become more manageable in the future, perhaps before we die. The internet and world trade, combined with the last several hundred years of world travel, has created a world culture. One of the biggest music videos in the world last year was from South Korea. I just watched an anime from Japan after reading a book from England. The concept of "the other," the concept that led to the above atrocities, is diminishing as we begin to understand one another. Our species has advanced more in the last 300 years than in all of human history before that. So are humans capable of horrors? Of course, I've seen the victims of such crimes myself. But, scientifically speaking, people are becoming more rational, and the world is becoming a safer place in which to sleep.