I blackmailed a guy who slandered me and I'm going to avoid them.
When you think about it, was what you did the right thing to do?
Yes, I never trusted this guy in the first place and he deserved what I did to him
This person shouldn't be saying negative things about you, or anyone else.  However, the fact that you blackmailed him in the first place is even worse, as this could have caused real harm.  If you can't make amends for what you have done, maybe it's best for you to avoid him.
I don't care if he dislikes me and I won't regret it
It's never easy to have someone slander you and it was wrong of him to do, but you should take your blackmail more seriously. Him liking you isn't the issue, it's that it's a more serious thing to do.