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.
You really don't know about Westboro besides the MSM label do you? They don't follow the Bible at all (not even their own interpertation). They are cultists known as Tachmonites who have their own scripture divided into two categories:
"delectable epics" and
"letters to heretics." The "delectable epics" (the term is the group's) are based loosely on Acts in the New Testament. The epics detail the Tachmonites' various protests against gays, President Bush, Elton John concerts, and the military and portray the Tachmonites alternately as invincible "super heroes" and defenseless victims of brutal rage. Some of the epics are in prose, and some in poetry. They believe Phelps is "the last prophet," with the power to determine who will be damned and who will be saved. They themselves, as followers of Phelps, also have the power to condemn souls to hell.
You should look up their scriptures and read them. It is deeply disturbing. They only put on a 'Christian' mask to the media, it isn't what they follow in their church.