I suppose this one needs no introduction; Fred Phelps runs the Westboro Baptis Church, which consists mainly of his family, and specializes in vitriolic hate. Being a near-Calvinist and taking the parts of Calvinism that even Calvinists don’t talk about that much at face value, Phelps’s other specialty is thanking God for natural disasters, child murders, deaths, 9/11, and pain. His obsession is of course homosexuality, and he hates everyone who hates gays less than he do (the WBC picketed Jerry Falwell’s funeral because Falwell didn’t unequivocally condone executing gays).
It is important to note that the WBC is not out to win converts. WBC’s expressed stance is that since who gets salvaged and who gets doomed is predetermined anyway, there would not be any point. They are of course generally repudiated by even the most hardcore fanatics and rightwingers.
You can read more about Fred here. You can read an interview with Fred’s estranged son Nate Phelps here (and you can hear his recent address to the Reason Rally here - it's good stuff).
One point worth mentioning: While the WBC is mostly a family-based cult, member Stephen Drain is not related to the Phelps clan by blood or marriage and was not born into it. Drain is the producer of the documentary “Hatemongers” about the WBC, but after making the documentary he decided to join the church with his family. He may currently look like Fred’s eventual successor.
Diagnosis: Possibly among the most hateful, vile persons currently alive in the US, but what distinguishes him from equally hateful people such as Bryan Fischer, Matt Staver, Tony Perkins and Scott Lively isn’t the amount of hate, but the rather honest absence of any attempt to hide it.