We often tend to classify people as black or white. The reality is often that they are actually a shade of grey.
We make up our minds about someone based on one or more events or actions and then find it difficult to change that view about them. Everything they say or do is seen through the black or white filter we have decided to use for them.
For example, someone says or does something to me that I think is grossly unfair, mean or bad. Forever, I will classify him as 'bad' or 'black'. In reality, that could very well have been a one off incident. The individual could actually be doing some really good things. It was just the context or the circumstance that caused the particular event.
On the contrary, someone did something good. We tend to associate him or her with good in perpetuity. Even if he or she does something bad later, we tend to justify the action with some possible reasoning.
Television and cinema often portray characters in black or white terms and we tend to follow this in real life too.
Individuals are rarely completely good or completely bad. Human minds are too complex to be easily categorized like this. All people are somewhere in between.