It reminded me vaguely of 'Honi soit qui mal y pense,' though I'm not sure it says quite the same thing.

Or is it perhaps someone's response to that? Context would indeed help.

If it's not a saying and you just want a translation, I might say something like 'A bad person thinks everyone else is just as bad,' or 'A bad person assumes the worst about others.'

As for a situation where this saying would be appropriate, you might think of some journalist criticizing a superb singer for his "condescending attitude" when in actual fact he is the one guilty of this kind of behavior.

In that case, either one of my suggestions would work.

"Schelm" hat zwei sehr unterschiedliche Bedeutungen:

1)

Thank you all for your input.
I had been looking for some pithy aphoristic equivalent, possibly by one of the usual suspects from Shakespeare to Bob Dylan... Am somewhat surprised that such an animal does not seem to exist.

Thanks again!

Since the German isn’t an established proverb, you could get away with coining a similarly half-elegant one in English:

Nice suggestion, Bion!

On second thought, let's change the "those":

I like that, pretty neat! Yes to "they" ...

Thank you!