I have NO IDEA where they come up with the justification for what they do, but I can CERTAINLY tell you where they go wrong.

Jesus, the Son of God...When he was here on earth he gave out ONE and only ONE commandment:

John 13:34-35:
A new commandment I give to you, that you love one another, even as I have loved you, that you also love one another. And they shall know you are my followers, because of your love.

Do you see anything about what Westboro is doing that conveys love?

I look at John 13:34-35 as God's divine "BS Detector", and mine is pegged when I look at what Westboro does.