Why do I want to talk badly about others? Why do I continue to think badly of them even when the evidence proves it incorrect?
The assumption in your question is ‘why do I think the worst of someone when they don’t deserve it?’
If we unfairly victimise someone it is usually because we need an enemy. Inventing an enemy happens when we feel bad about ourselves (guilty, out of control, under pressure, etc) in a way that we cannot take ownership.
Having an enemy gives us reassurance that we did all that we could do, but the presence of this other person (or group or state…) means we are unable to be as perfect as we would otherwise be.
Hence we are stuck in victimhood not through our own actions, but because of others.
It is the ultimate denial of accountability.