The logic error is in thinking that a single abusive prison guard necessarily means that all prison guards are inherently bad. That's the same collectivist thinking as a racist.
Many prison guards are inherently bad. If they had different character, they probably wouldn't do that job.
I'm not disputing that. That's not a good reason to eliminate prisons. The proper question seems to be - how do we collectively hire good/moral/fair/humane prison guards?
Irrelevant to the discussion. We're talking about how to morally deal with people who refuse to conform to the larger societies agreed upon rules. It's not as simple as *do nothing*.