I was mean to this man
Why were you mean to him?
I saw him flirting with my girlfriend and I didn't like it.
While seeing someone flirt with your partner is indeed annoying and frustrating behavior, going out of your way to be mean to that person back is inappropriate. You should talk with your girlfriend so that she knows how you seeing her reciprocate someone being flirty is hurtful to you. That will prevent that behavior in the future and you won't have to be mean to others.
You need to chill out. I was just joking around.
It is kind of mean to pretend to joke about this when it happened. If you are overly jealous your girlfriend might be embarrassed or turned off by you.