I've seen the "explanation" given by the person who initially nominated him:
Professor John Hudson of St Andrews University attacked the medieval martyr as a "founder of gesture politics" and "master of the soundbite".
Rather than serving as an inspiring role model, Becket was "greedy" and could be viewed as "hypocritical", says Professor Hudson, who nominated the archbishop as his biggest villain of the 12th century.
"Those who share my prejudice against Becket may consider his assassination ... a fittingly grisly end," he says.
This all seems pretty... vague, I guess? And sort of childish, basically, when we consider that it puts Becket on the same list as the likes of the fascist demagogue Oswald Mosley and the serial killer Jack the Ripper.
The article goes on to say that the thousands of readers who decided the poll may have voted for Becket because he was the only one they had heard of, but how likely is that? And how accurate or meaningful are this John Hudson's claims about Becket in the first place?
Yeah, I can't see it. Every claim I come across seems like a pretty strained reading of history.
There's actually quite a lot of literature of Becket actually being a role model in the middle ages. See, inter alia:
Roberts, Phyllis B. “Archbishop Stephen Langton and His Preaching on Thomas Beckett in 1220.” In De Ore Domini: Preacher and Word in the Middle Ages, edited by Thomas Leslie Amos, Eugene Green, and Beverly Mayne Kienzle. SMC 27. Kalamazoo, Mich: Medieval Institute Publications, 1989.
Roberts, Phyllis B. “Thomas Becket: The Construction and Deconstruction of a Saint from the Middle Ages to the Reformation.” In Models of Holiness in Medieval Sermons: Proceedings of the International Symposium (Kalamazoo, 4-7 May 1995), edited by Beverly Mayne Kienzle, 1–22. Textes et Études Du Moyen Âge 5. Louvain-la-Neuve: Fédération internationale des Instituts d’études médiévales, 1996.