I wish I was meaner to someone after the fact
Why would you want to be meaner?
They were bad to me so I should have been worse to them.
It's frustrating to deal with bad people, but that's really not an excuse to be mean to them. People might see you as the problem if you're being mean. In the future just try to limit your interactions with the people.
So I should give more respect to bad people?
Just let them be bad without you yourself becoming bad. There is no point to stoop down to their level. Just be who you are.