How are they following basic tennants when they have their own scriptures (not the Bible) and consider Phelps the final arbiter on sin and they themselves have the power to condemn souls to hell?
They follow their interpretation of the Bible. Phelps and his crew have never said that they can condemn people to hell. People are headed to hell because they violate what Phelps and his mob believe are God's teachings. From their point of view it's God who is condemning them, Phelps is just pointing that out to them. I suppose that they consider themselves being helpful by doing that.