I'd take what was probably the deadliest killer and gunman America ever produced........Skinny, scrawny, baby-faced, stone cold killer Audie Murphy.
I just finished reading "No name on the Bullet" the best biography written about him. After the war, he was ALWAYS armed, and ALWAYS ready, willing, and certainly able to kill.
Most people never knew that Murphy was so fast with a gun, ANY GUN. There were a few people around who were as fast as he was, but NOBODY was faster. Unlike most of the "fast guns", Murphy was that fast with live ammo. Many people saw him make shots that just defied belief.
It also helped that he killed between 250-400 German and Italian soldiers. Any number of "tough guys" took one look at Murphy's eyes and suddenly remembered that they had important business elsewhere.
One astute observer recommends that the next time a Murphy movie is on, wait for a closeup and take a look. Murphy is usually sardonically smiling, but his eyes are like a shark's, absolutely dead.
Other than Murphy, I'd go with Wild Bill Hickok or John Wesley Hardin. These guys were great shots, but were so dangerous because they were ALWAYS ready to shoot.
Point a gun at most people and they're thinking about dying. Point a gun at them, and they were thinking about how they were going to kill you.