Their pain is unbearable
A vindictive person has misguided pain. They feel frustrated, helpless, hurt or ignored and are unable to change their circumstances without ensuring that they affect others in the meantime. They don’t have the necessary strength inside to find better ways to handle their feelings.
Instead, they lash out and convert pain into anger and seek revenge by taking that pain out on others.
Most commonly, they want to bring others down with them. They feel by using the power of manipulation, they are able to not have to experience the misery by themselves – they can in fact bring others in. They are the core of toxic behaviour in the workplace and contrary to many stories you read, it often is someone in a team, and not the manager or leader who is usually too busy to notice what is really going on.