OYF was undoubtedly the most ruthless of the 5 Greats, but was his reputation for 'Evil' due to his personality or racial discrimination from Wulin?
OYF was certainly unscrupulous in his methods and never missed any opportunity to take advantage of his opponents, whether fair or unfair but was this enough to label him 'Evil'? He never established a school/cult with the intention of Wulin domination, in fact he seemed indifferent to that sort of power. Although he allied with the Jin's, this was an alliance of convenience and he never did anything openly against the Song. He was not squeamish about killing innocent people but did not make a habit of it or do it on a big scale.
We know that OYF came from the far West, possibly outside of Han China proper and may even have had red hair. Could his exotic appearance and background contributed to his 'Evil' reputation? Wulin was infamous for its racial discrimination, so it would be easy to see why they would make the most obviously 'foreign' of the Greats the 'Evil' one.