I’m focusing on hands-on guys here, in the modern era.
The actual winner is Vasili Blokhin, Chief executioner of the NKVD (Soviet secret police) during WW2, with about 22,000 dead by his German pistol (used for reliability and deniability).
But one of the runners-up actually has a good-guy, Travellerque feel to him.
My best bet would be William E. Fairbairn, an englishman who survived more than 600 street fights in Shanghai, one of the most, if not the most dangerous city in the world at the time he was on active duty at the Shanghai Municipal Police Department, during a 33-year period (from 1907 to 1940).
Thugs and ruthless criminals there preferred to murder policemen instead of letting them stay alive, because they would certainly face death penalty for their dirty deeds if caught.
The following isn’t a fact nor a deep scientific approach, but I’m quite certain that of those 600 life-risking violent encounters, in which Fairbairn made good use of his techniques learned by practicing diverse martial arts, knife fighting, and shooting, and with the belief that there’s only one rule to escape from death: Kill or get killed, himself, if didn’t disable his opponents, might have killed about 400 of them (just a rough number, having in mind that thugs most of the times didn’t come alone looking for a fight). This number might vary depending on the number of criminals involed in each encounter, so the cipher might go up as high as 600 (and perhaps even more), or as low as 300 (something unlikely, knowing they all were kill-or-get-killed encounters). In the end, sadly, almost everything relies on speculation.
His techniques didn’t follow the Queensberry rules (that is, they were out of every bit of gentlemanry), as we can find some nasty ones, like testicle squeezing, sternum-breaking hits, eardrum breaking by vigorously clapping ears, and of course, artery-slashing strikes with his co-designed Fairbairn-Sykes fighting knife (in one of his books, you’ll find a table with information about estimated times to one foe to reach inconsciouness, and even death, depending on the artery cut).
Facing death hundreds of times (and getting alive from all of them) came with a hefty price: His body was covered with hundreds of scars from his chest to his ankles, not to mention the occassion in which he got a dozen of stabs while starting to serve at the Shanghai Municipal Police Department, at a violent encounter with Chinese thugs, who left him for dead, and made a drastically change in his life (since he knew next time, he wasn’t going to be so lucky).
Despite having gone through all these heart-bursting experiences, he was known to be a polite, simple family man, with an unwavering moral compass, and one who never uttered profanities, boasted of himself, or drank, which, in my opinion, made him a one-of-a-kind individual.
Some of those Victorian-style Imperials were pretty tough!