I'm pissed that my ex is dating my friend
Why are you upset about your ex-girlfriend dating your friend?
I don't want my ex to be happy
Since she is your ex now, she is free to date whomever she wants now. Even if it's painful that it had to be your friend, love blooms in mysterious ways and it's unkind to treat her like she is still yours.
I don't care about what other people think. I'm going to tell my friend to dump her.
I understand all the hurt you might be feeling, but I think you really should try to stay away and calm down. Everyone deserves to be happy, even you, so trying to intervene in the relationship will only further bring everyone down.