One of my pet peeves (I call it "Spot") is when people treat others unfairly because they don't like them.
It's natural that, when you don't like someone, everything they do rubs you a bit the wrong way. A minor annoyance that you'd instantly forgive in a friend is yet another piece of evidence against the unliked person. But I really try to refrain from criticizing someone I don't like for something I would never criticize a friend for. For instance, I don't make fun of people for being fat, or unattractive, or bald, or old, or whatever, so it's not OK to make fun of those things in someone I dislike.
But what I really hate is when this happens at work. One woman that I work with becomes totally suspicious of a person's every move once she takes a dislike to them. The disliked person will visit me with a perfectly reasonable request - something they need for their job - and I'll later be asked, "What did he want?" in a suspicious way, and interrogated about why he needed that thing, and told how he didn't really need it, which will be followed by a reiteration of how & why she doesn't like him.
Even the most disagreeable coworkers often legitimately need things from us in the course of business. It's just dumb to be continually suspicious, intransigent, and difficult towards them. And it sucks.