In terms of understanding why people bully others, it's too simplistic to say that bullies have low self-esteem. It is true to say that happy people don't tend to intentionally hurt others - put simply, hurt people hurt people. But even that is too simplistic, not all people who have been hurt go on to bully others.
The key factor is not that people have been hurt, whether that be via trauma and abuse, or emotional neglect (which can happen even in very wealthy families). The key is how people manage that distress, and in the case of bullies it's largely via what therapists call "project and attack".
If - as well as having lots of feelings to manage - we learn from our family, our experiences or our environments, such as schools or peer groups, that vulnerability is a weakness, that strength is valued and frailty is to be judged then we're likely to reject that vulnerability in ourselves, and then also attack it in others.
The vulnerability - or insecurity if you prefer - can be extremely well hidden, invisible even, but we know it's there simply because if it wasn't then the bully would have no need to tear others down.
In its most extreme forms you might hear this referred to as "narcissism", but we don't need to bandy about clinical terms to understand the dynamics at play. Empathy is diminished as a way to not feel pain and others' pain is used by the bully as a way to attempt to elevate their own standing. "See, I'm not weak and pathetic like you."
It is of course true that this sort of entitled cruelty is culturally bestowed upon those of wealth, and privilege, but it is more an amplifying feature than a direct cause - not all wealthy Pākehā men are bullies. And bullies come in all shapes, ethnicities and genders.
So how do we truly know if someone is reformed, how can we trust an apology? Can a leopard change its spots?
Well, this approach to our own vulnerability - to project and attack - exists on a continuum, from the mildest of mean friends to the most flagrant narcissist, and the severity of the behaviour is a good predictor of how likely change is. But ultimately for change to occur, the person must genuinely find ways to connect with and tolerate their own vulnerable feelings.
They must find ways to value being kind in the face of vulnerability, and fully experience their emotions - shame and fear in particular.
This can happen in a multitude of ways. Simply growing up can be enough, or other major life events, becoming a parent, falling in love with someone who can give you hard feedback. Or being forced to confront oneself because of adverse consequences via the law, employment or losses caused by one's behaviour.
And apologising can be important, but sorry is, of course, only a word. And an apology that is motivated by the bully's desire to alleviate their own guilt, to make themselves feel better, is not really an apology. It's just more of the same - using others for emotional gratification.