Abby is well known for being very nasty to people. She doesn't seem to have many friends at Capeside High, eventually befriending Jen Lindley in Season 2.
When we first meet Abby in Detention, she already has a reputation for being horrid among the gang (minus Jen, who has never met her before). She comes off as a kinda bored, instigating teen who revels in toying with other people's lives.
This perception of her is only brieftly altered with Jen in the periods where she spends time with Abby (which usually coincide with Jen feeling left out of the gang).