Bullying
Written after reading OoTP. R/R.
Severus Snape had just found out that Harry Potter would be starting at Hogwarts, and sure enough, Snape wasn't thrilled. His father, James Potter, had made Snape's life a living hell for the years they attended school together. As Harry Potter entered his classroom for the first time, Severus made a plan, to ruin Harry's life like his father had ruined his.
During that Quidditch Match in Harry's first year, Severus didn't change. He saved Harry from a fateful death, but still went back to hating him. Now, we flash forward to the fifth year, where Severus had accepted teaching Mr. Potter Occlumency. He didn't owe it to James, but to Lily, to teach Harry how to protect himself. Part of Snape wasn't thrilled, but who would be, teaching their childhood rival's son to protect himself.
That day, when Harry found that pensive, Harry unlocked secrets that Snape didn't want him to know of. As the memories ended, Snape sadly kicked Mr. Potter out.
"I'm so sorry, lily. I failed you. I can't help Harry, he's a lost cause. All this bullying because of the torture that his father put me through? Is it really worth it? Taking my anger out on a kid?" And for our resident Bully, it was. It was revenge, sweet revenge. James Potter made his life living hell, and it was time for payback.
The end.