It's hard to explain, but to try to sum it up: We tend to think the best of people we like and give them the benefit of a doubt with whatever they do, but with someone we don't like we do the opposite, and tend to assume the absolute worst of intentions with everything they do.
For example: If someone we like likes Thing X, we think "Oh, well of course they like Thing X! Thing X is awesome and so are they!", but with someone we don't like, it's more like "Ugh, of course they'd like Thing X, fuckin' asshole.".
Like with people we like, we're more willing to overlook their flaws, while with people we don't like, even if said flaws are basically the same, we only focus on them in their case.
It's kinda like that Parenti quote about capitalists always assuming the worst with everything communists do, even if it's the most benign shit in actuality, but applied to individuals instead.
Not trying to make any sort of larger statement political or otherwise really, or to act like this is a groundbreaking discovery of the human psyche, this is just something that's been on my mind for a while and wanted to get it out somehow.
A psychology phenomena you might be interested in is the Fundamental Attribution Error. It’s when people tend to attribute personal qualities to other’s actions rather than material reasons. The classic example is “They were late to work because they are lazy” vs “They were late to work because there was a lot of traffic.”