It really depends on the definition of 'Wild West'. Especially as to area and time frame. All of the men mentioned (with the exception of william Bonny, Billy the Kid) were products of the Civil War, which ended in 1865. Most of the men were products of the frontier in Kansas, rather than what is thought of as 'The West' today. They , also, were active throughout a relatively short period, say 1870 to 1890. So... by way of that explanation, I would nominate none of these but rather "Bloody" Bill Anderson. Anderson was the dreaded scourge of western Missouri and eastern Kansas during almost all of the Civil War. He was either a blood thirsty renegade and pschyopath or a man bent on justifiable revenge. At any rate, his killings were probably in the hundreds if not thousands and these consisted, largely, of men (and some women and children) that were simply in the wrong place at the wrong time. A great deal more is written about Quantrill, with whom Anderson was informally associated, but historical accounts show that, as far as ruthlessness is concerned, Anderson was by far the worst...