“Doing bad is easy but doing well is not”. Yes, that saying is the most concrete explanation why good people do bad things. Good people do bad things with good intentions. We, humans, are all sinful by nature. No one of us can behave perfectly and sinless. But we have the capacity to avoid temptations and to distinguish what’s right from wrong.
Good intentions cause most of the world's great evils. There are many things that happened in the past that can construe these words. Like for example, Communism, the worst mass-murdering ideology that happened in the past, was a belief supported by many people who believe in human equality and social and moral progress. It took a while for them to figure out how evil Communism is. See, one of the problems is ideologies. People often have a strong sense of being right. This "sense of being right" is the problem. Once people believe without a doubt that they are right about something, they can do bad things and feel justified in doing so. When it comes to personal and social decisions, intending to do well is nonsense. The heart is an unpleasant guide to do well.
In order to do good to yourself and in order to fulfill our obligations as a member of society, what humans need to achieve more than a good heart is wisdom. If you really want good to prevail, the answer is wisdom, not the heart. So, why do good people do bad things? Because they lack wisdom. Without wisdom, you can be nice and kind, but you will not do nearly as much good as your good heart would like you to do.
* http://www.creators.com/opinion/dennis-prager/when-good-people-do-bad-things.html * http://thecodersperspective.posterous.com/why-do-good-people-do-bad-things