Good people can do very bad things given the situation and system in which the situation occurs.
In the summer of 1971, the police publicly "arrested" 10 students who did nothing wrong besides agree to participate (for $15 a day) in one of social psychology's most notorious studies since Milgram's house of pain back in the 60s. These students were taken to the "Stanford County Prison" (a mock prison located at the university) where they were set to be held prisoner for the next two weeks.
Also participating in this "experiment" was 11 students who, by the luck of a coin flip, were to play the role of prison guards. According to the head researcher and acting prison warden, Philip Zimbardo, "The purpose [of the experiment] was to understand the development of norms and the effects of roles, labels, and social expectations in a simulated prison environment." What followed was an unimaginable series of events that appeared to bring out the worst in both the prisoners and the guards. However, this "evil," argues Zimbardo, was not the result of the students who were chosen specifically based on their moderate demeanor, lack of prior trouble, and representative status to their peers, but the "evil" was a result of the situation and the system in which the situation occurred.
The experiment was stopped after only six days due to extreme adverse psychological effects of many of the participants. Zimbardo succeeded at demonstrating the power of the situation.
Zimbardo was strongly criticized for his role in the experiment and his lack of objectivity by making himself "superintendent" of the mock prison. This study was conducted at a time when APA ethical codes were far less restrictive than they are now, so Zimbardo did not receive any disciplinary action. In addition, the nature of the experiment made it difficult to have controls in place putting the scientific integrity of the findings in question.
We tend to think that we cannot be influenced by the situation, but we can. There is a strong American tendency to attribute "good" and "bad" to one's disposition, rather than accept the power of the situation and system that can strongly influence behavior. The findings of this experiment, as well as the concepts this study dramatizes, are crucial in understanding morality and justice.