Sentence:  Taint is here a metaphorical expression. It means literally something which <WSD>tinges</WSD>. "The mean and malignant passions" are therefore, first of all, a substance which <WSD>tinges</WSD>. This substance which <WSD>tinges</WSD> "will creep." 
Meaning of the word marked with <WSD> tags:  To change slightly in shade due to the addition of colour;  to change slightly in quality due to the addition of some other thing.