‘The Shadow Knows’
Precisely, The Shadow knows “What evil lurks in the hearts of men.”
I can’t believe this was a radio show. I would’ve sworn “The Shadow” was a television vehicle when I was growing up back in The American Pleistocene (what a book title!).
Alas, it was a radio show, dating me beyond what I even remember.
As for The Shadow, he was played by Orson Welles and others, but here goes — and I beg you to prove that evil doesn’t lurk within all of us, maybe nascent but there nonetheless: