There's nearly always someone in any walk of life who unwittingly sticks their head above the parapet and screams "abuse me". Sometimes they actually try to shrink below the surface in the hope of avoiding said abuse but generally to no avail. It's hard to put your finger on exactly why they command this level of hate but, generally speaking, it comes down to the nature of us humans. Just as there always emerges a "leader" among a group of cohabitants, there inevitably emerges a victim as well.
It's not always the obvious target either. Think back to the school yard; yes, it was easy to pick on the ginger but often he or she had an endearingly cheeky streak to them plus they could be wildly unpredictable and prone to rage, thus making them a valuable playground ally.
The fat kid was like a red rag to a bull when it came to hurling abuse, but it was so obvious and easy it lacked gratification on the part of the abusee. Chubs was also often accompanied by an equally large brain so was good for copying homework from and to sit beside in class, provided he hadn't been running round too much at playtime working up an awful sweat chasing kids for their lunch money. He could also be a good scrummager.
The smelly kid was inevitably a struggler when it came to academia and was forever haunted by the time he wet his pants in Year 1. He usually wasn't worth abusing.
No, the ones who commanded the most attention were those who were of a duplicitous nature. They could be valuable on the one hand but wouldn't hesitate to tell the teacher if you'd been fighting at lunchtime. They would let you join in their game of cricket but wouldn't help you with some insufferable mathematics. They may have had the best toys, but wouldn't necessarily share them. They could be friendly and untrustworthy, which made you wary of them, hence their propensity to be cast in the role of villain.