I still feel guilty about laughing at a woman almost 16 years ago
That's so long ago. Why, what happened to make you laugh?
She stabbed her boyfriend after she was laughed at by him
It's admirable that you're remorseful over it. It was 16 years ago, though, so I think it's fine to let it go.
I still feel quite guilty. I shouldn't have laughed and I feel like  a bad person. I wish I could go back in time to change my reaction.
It's true you definitely can't change anything, but you can only move forward from it. Continually mulling on it, I believe, only hinders your happiness. You had enough remorse, so why not seek joy now?