Sometimes it is faster burning, this is the adult version of a tantrum.  The hate is short-lived in the moment.

Other times, it is slower burning, something happened to the person, and depending on the event and subsequent training:  the person may act without others being aware of the intensions to harm others whenever it is necessary to get what they want from them.

Usually, we call the above, loosing control over our better selves.  The base animal instincts to survive are much cruder than the higher instincts to live being our best selves.

Sometimes the dark side wins and we slide into criminal behaviour.  The longer we survive like a criminal, the more likely we will join this kind of society.  It is full of dog eat dog behaviour:  lying, cheating, steeling, fighting to inflect fear, anger, physical harm, wonton destruction of materials, eventually harming people enough to persuade and manipulate them into submission.

Being hateful only gets sicker beyond this level.  Education is the best medicine.  As counter productive as it seems on the surface, the more we know the healthier we potentially become.

It is always best to forgive and forget.  Obviously, a hard lesson to learn.