He could fire off 10 shots because he was ambidextrous. Revolvers are carried with the hammer down on an empty chamber lest they go off accidentally and put a big hole in your leg
Here's a good video explaining why people would load post-Civil War single action revolvers with 5 shots instead of 6: https://www.youtube.com/watch?v=ldHPNnsp-cs.
Even today, in a world of very high quality modern firearms, safety issues arise. The Sig 320 - now the M17 service pistol of the US Army - had a problem where if dropped just so, it could discharge. It was kind of a scandal; modern firearms are incredibly safe to handle, compared to even 30 years ago. Consider that in the 60s it was not unheard of for pistols to require the user to pull the trigger as part of operating the decocker mechanism. Insanity to modern eyes.
Anyway, firearms at that point were kinda new. We didn’t know what “safe” really meant, we didn’t do things like drop tests, we lacked the material science to make carefully heat-treated components, we lacked the manufacturing ability to make the sorts of small, high quality components we use today ... it wasn’t a design defect as much as we didn’t have the science and understanding.
The trade-off is between unwanted firing and failure to fire when wanted. Which risk do you suppose your shootist customer would prefer?
And in any case one solid hit on your opponent, and there's a pretty good chance whatever shots he manages to get off after that are probably not going to be the most most threatening / well aimed.