There is a saying 下衆の勘繰り(Gesu no kanguri) in Japanese.
"Gesu" means a person who is mean, lowly, or base in his/her heart. "Kanguri" means interpreting falsely someone's intention as bad.
A translation of this saying can be: A mean person often interprets an innocent person's intention as bad."
Is there a similar saying in English? If not, what would you say to such a person?