Like Mal said...Robert Mitchum in
Night of the Hunter...scariest and most despicable movie bad guy ever.
Then there's Richard Widmark in his first film,
Kiss of Death...pure evil with the most insidious giggle ever.
There's a little known film named
Caught (early '50's, I think) starring Robert Ryan as the insanely domineering husband of Barbara Bel Geddes. His performance is chilling.
As for
Taxi Driver, I think Harvey Keitel's character was more disturbing than Travis Bickle.
How about Alan Arkin in
Wait Until DarK?
Mr. Brocklehurst in any film version of
Jane Eyre, but especially the 1940's one starring Joan Fontaine...don't know who played him but he's an asshole
Some of the most evil women...
Bette Davis in
Whatever Happened to Baby Jane?...
serious childhood trauma
Faye Dunaway as Joan Crawford in
Mommie Dearest
and in similar femme fatale roles...
Barbara Stanwyck in
Double Indemnity
and
Kathleen Turner in
Body Heat