Does it make you angry when someone speaks badly of your child? Does it make you sad? Do you hurt for your child?
How do you think God feels when someone speaks negatively about one of His children?
Every time you say something negative about another person, no matter who you are saying it to; you are talking about God's child. It doesn't matter if its true. It doesn't matter how angry you are.
It doesn't matter if YOU feel justified in doing so.
God doesn't see it that way.
I didn't make that up. It's in the Bible. Really.