I'm going to leave a mean comment on a former friend's social media
Why will you leave a mean comment?
They were mean to me.
You shouldn't bully your former friend on social media. That's horribly childish.
It's so unfair that they said something bad about me and I should walk away. They deserve a negative comment.
It was wrong for them to bully you like that online and it is wrong for you to do the same. People will assume you are also a bully, they won't think that they deserved it.