Frank Horrigan doesn't qualify as Pure Evil. Heck, I don't even know if he was proposed ever, even in the discussion rather than blog.
Who is he?[]
Frank Horrigan is the secondary antagonist and final boss of Fallout 2. He is a super mutant who is one of the Enclave's most powerful pieces of weaponry by himself, given his brutish effects from FEV and power armor. His personal loyalty to the Enclave and his extensive simple-mindedness has made him do some pretty bad things, for sure.
Why doesn't he qualify as Pure Evil?[]
Frank Horrigan does not qualify for a variety of reasons. In Fallout 2, he has a backstory of having had been accidentally exposed to the FEV (Forced Evolutionary Virus), which given his already low intelligence and high loyalty (but no brutality given), only increased the degree of the two. As a result, the Enclave did a lot of experimentation on him and more or less used him as a weapon. Given this, one could consider him a Villain by Proxy, because of how the psychological effects of FEV are beyond Frank Horrigan's control; thus being taken advantage of, and little backstory given prior to his infection. Frank Horrigan also deep down was patriotic and cared for the President (namely Dick Richardson, the main antagonist of Fallout 2), and showed such by saying a powerful oath seconds before death.. If he were Pure Evil, he would only view the Enclave as an excuse to commit heinous crimes rather than be patriotic and protective. Although this doesn't give him the mitigating factor of being an Extremist, it at least shows a sign of care.
His moral agency is also not made clear enough throughout the story of the game. He did indeed commit heinous crimes for the Enclave, such as genocide, brutality, and silencing of opponents via death, but it wasn't made clear his true motive for carrying out such.
Final Verdict[]
Not Pure Evil, and even with mitigating categorization being Villain by Proxy, and Protective (if not, at least Guardian).