I had written about this topic back in 2008. Every individual is very complex. It is impossible to classify someone has completely black or completely white. Everyone is somewhere in between - a shade of grey.
Despite realising this, I find myself (and others around me) bucketing people into either black or white. If someone has done something bad, I immediately assume that he is a bad person. If someone has done something good, I immediately assume that he is a good person.
The truth however is that it is merely the action that was good or bad, not the person himself or herself. All of us do good things and bad things. We tend to be less generous of others than we are with ourselves.
This colours our perspectives in very biased ways. If someone whom we have classified as 'bad' does something good, we look for some selfish motive behind that act. On the other hand, if someone we've classified as 'good' does something wrong, we explain it away by saying he or she might have had some compulsion.
We must try to separate the actions from the individuals.
Some people go to another extreme and generalise entire communities, religions and countries in this manner. If they've come across a few individuals from a community doing something, they will generalise and assume that the entire community is like that. At this point, the principle of collective justice is inevitably invoked. For the actions of one person in the community, the entire community must be punished!
In reality, it was one action from among many other actions that caused us to perform this classification. How dangerous this kind of classification can be?