What actors/actresses always play really good bad guys?

Do you mean actors who always play bad guys and who always do a good job at it, or actors who, on those occasions when they do play bad guys, do a really good job at it? Your title suggests the former interpretation but I’m not sure if the example in your OP fits. John Lithgow often does non–bad guy roles, some of which have been highly popular and acclaimed (e.g., Dick Solomon on 3rd Rock from the Sun).

Walter Brennan

Lee Van Clieff

Eli Wallich

Jack Elam

Bruce Dern

James Woods
Sorry about the spelling.

Subtle, and funny!

And lost.

He was amazing in Deadwood.

Louise Fletcher.

For years I only knew him from the affable tv series Lovejoy, where he plays an antique dealer surrounded by other affable characters. That and a nuclear reactor technician in Space 1999.

None of which prepared me for the calm menace of Al Swearengen.

Having seen Sterling Beaumon play a very young spree killer on Criminal Minds (an episode that had the biggest jaw dropping surprise ever)and a young serial rapist and killer on Law & Order: SVU, he has a good future playing really bad guys.

Viggo Morgenstern also plays either a great baddie, or a great hero

Danny Trejo.

Walton Goggins. Walton Goggins - Wikipedia

He’s one of those kind of fuckeers that you can’t wait to see get a bullet in the eye. Sort of a new Bruce Dern.