Victor Mature, someone who i didnt used to like too much but have a newfound appreciation for these days. His performance in "Kiss of Death" is very effective!
An absolutely awesome shot of Richard Widmark, to me a very underappreciated actor, I mean, who can play a giggling psychopath better, you tell me! He is completely unforgettable in "Kiss of Death", "Roadhouse" and "Pickup on South Street" (to name just a few) but was also excellent in several enjoyable westerns and war films as well.
Speaking of a guy who was adept at playing a psychopath, here's Lawrence Tierney! I've had a special admiration for this guy ever since i saw his great performance as "Joe" in Tarantino's "Reservoir Dogs", so after that i had to go and discover his early films like "Dillinger", "Devil Thumbs a Ride" and Robert Wise's ultra-hard-boiled Noir "Born to Kill". Tierney convincingly plays a psycho killer in all 3 of those!