Richard Dix plays artist Ralph Harrison
The Secret of the Whistler on IMDB
"I am the Whistler and I know many things, for I walk by night. I know many strange tales hidden in the hearts of men and women who have stepped into the shadows. Yes, I know the nameless terrors of which they dare not speak."
Artist Ralph Harrison is married to a wealthy woman who has been suffering heart attacks. He begins an affair with an artist’s model who is only interested in his wife's money.
His wife discovers the affair and threatens Ralph, saying she’s going to take him out of her will. So, he decides to poison her, with her own medicine, before she can meet with her lawyers.
read more on Wikipedia