We've got a proverb that could be translated as "The thief believes that everybody else is the same", meaning that if you have a negative trait of character you tend to believe that everybody is going to have the same trait and act accordingly. We're trying to find an equivalent English proverb or idiomatic expression, but I can't for the life of me remember if there is one or what it is... can y'all help? Leftpond and rightpond expressions will be equally welcome.

Thanks!