I seem to remember that in the middle ages there where some people that argued that redheads were evil, unthrustworthy, ...

There are many examples of redheads being stereotyped as untrustworthy in medieval times. The "Proverbs of Alfred" warn not to choose a red-haired person as a friend and the "Secretum Secretorum" warns against using redheads as advisors. Another manuscript, from the 14th century, notes that redheads are rarely faithful in friendship and a work published in 1659 denounced the "vulgar error" of "censuring red-haired men."

Not sure this is 100% solved yet though.

Source: http://www.themythsandhistoryofredhair.co.uk/heresy.html and own experiences.