There are numerous cases of humans engaging in really depraved, sadistic, malicious behavior. Seriously there is a LOT. But there are other factors like wealth, political power, intelligence, connections, etc. Many factors determine the difference between a path of torturing people in one's basement or trying to genocide whole civilizations with bioweapons. The common denominator is of course the malignant psychology. The teenager out there somewhere torturing a cat to death right now isn't an alien, and if he had the power, money and intelligence to "see what happens" introducing new bioweapons he would go for it. This isn't to say that such evil lurks in everyone, it doesn't. The thing is humanity has a truly miraculous level of variety versus any other species on the planet.