A reformed bully returns for his school reunion, falling into a deadly trap, as he and his friends are hunted down by an old victim.

When his best friend is killed at their 10 year reunion, a reformed bully must stop his old high school victim turned killer, before he murders again.?

Maybe... Although the name of the victim turned killer is known, so there's no mystery there.

"When his best friend is killed at their 10 year reunion, a reformed bully must discover which of his old gangs high school victim's revenge has turned to murder."

or (without the word 'revenge')

"When his best friend is killed at their 10 year reunion, a reformed bully must discover which of his old gangs high school victims has turned to murder."

or (without 'old gang')

"When his best friend is killed at their 10 year reunion, a reformed bully must discover which of his high school victims has turned to murder."

Hope that helped, good luck with this!