When discussing the idea of evil one first has to recognize that the very idea of being "evil" is a purely human trait. For example, a human mother killing her child would be considered awful; yet a mother rat eating its own child rat is simply part of the food chain (source:
http://www.ratbehavior.org/infanticide.htm). So when studying the idea of evil, you first have to come up with a clear definition.
In regards to the Zimbardo experiment (and I strongly advise you to research the Milgram experiment as well:
http://en.wikipedia.org/wiki/Milgram_experiment.)
Assuming we can all agree upon a relative definition of evil, there still remains the question: what makes people evil and why do they remain evil? As most questions in psychology must be addressed, there are a large myriad of answers. For example, a child may grow up witnessing extreme domestic abuse, which he may model later in his life, leading him to beat his wife. In this situation, his evilness seems to be a direct result of bad parenting. A lack of critical thinking skills and empathy also would seem to play a part here.
But, as the experiments show, people can be evil for many other reasons, and some will be evil simply when they have an excuse to. It is a broad topic and you could pick one of hundreds of viewpoints to discuss it from. Anyways, here are my answers to your questions:
1.What are your opinions on this experiment?
It is interesting because it examines how people behave when given certain roles that naturally encourage submission, dominance, retaliation and violence. I am glad it was conducted as it also raises hundreds of prevelant questions. It is fortunate it occured in the 1970s, during still early, experimental phases of psychology, as this sort of experiment would be considered grandly unethical today.
What do you think was gained by it, if anything?
Since it was halted early, and no strict scientific evidence seems to have been gathered, the greatest contribution of said experiment is the questions it raised about morality, values, ethic and the roles we fall into.
Do you believe it was unethical, ethical, worth it?
It turned out to be unethical, which no one at the time could have precisely predicted. From an ethical stand point, it is good that it was halted early, though who knows what we would have found out about human nature had the experiment continued over its intended course. And yes the questions it raised is worth 6 days of unhappiness from many of its participants.