Yeah this guy violated some basic rules and compounded them by trying to carry on conversation thereby not being completely cogniscent of what he had, i.e., a loaded revolver with the hammer cocked.
I've noticed myself that after firing several cylinders D.A., if I decide to cock the hammer and fire S.A., my trigger finger has developed a memory and I have to be aware of that.
Not excusing this incident whatsoever, but I could see it happening if you fail to follow basic safety and are distracted.