English equivalent of "a**holes don't have horns"

I suggest this old saying by Tucker Max "How were you supposed to know he would stoop so low? After all, the devil.......

Tucker Max - an American author and public speaker.

Area, the only phrase I can think of that's like this is, you sometimes hear variations on "sign on their head" or "sign hanging over their head" or "sign over their head..."

So, "assholes don't go around with signs over their head" or "I wish idiots had a sign over their head alerting me to the fact they were idiots.." sort of thing.

As I mention in a comment, it's absolutely understandable. And, I'm pretty sure I've actually heard someone say something pretty much like this (using the word "horns") in English. So, you know, something like "the devil doesn't go around with horns and a tail!"

{Footnote: My grandparents etc are Scottish and I get some subtle linguistic clues from them. it's just possible there is some connection there since, as you know, there are many connections to Scottish usage, accent, etc in the subcontinent. perhaps there's a scottish saying along the lines of devils not always coming with handy visible horns (that would be typically miserable of scots grandmothers!)}

Shakespeare said that someone (Iago, I think) "could smile and smile and be a villain", also that "There's no art to find the mind's construction in the face." (Macbeth) Can you use that?

The Hindi expression is a good one, we should steal it.

Why not Shakespeare? In Othello Iago says:

But I will wear my heart upon my sleeve

that is, he claims to display his emotions openly, but of course he does quite the opposite. If you explicitly want a slang expression (something modern), Shakespeare actually made it to the urban dictionary. Since the Hindi expression also uses a negative construction you could say something along the lines of: people don't wear their hearts on their sleeve. Or, if your English speaking friend is familiar with Shakespeare's work, you could just call the treacherous, stony-hearted devil incarnate - Iago.

P.S. If you want to offer words of comfort, after a while saying that this person's leaving was good riddance might be the right phrase.