Yeah he pulled the trigger

Yeah --- saw that--- I wouldn't allow that "expert" near a weapon

With a Colt SAA, There are 2 positions for the Hammer, 1/2 cock for loading, and unloading,

AT the 1/2 cock position it is hard to pull the trigger, so you don't shoot yourself while loading

SOO you get the gun loaded, Hammer down---- Nowadays that's safe, BUT the old timers used to put the hammer down on an empty chamber

OK so hammer down and you're smart-ass Alec fucking around with the gun, He pulls the hammer back, NOT far enough to engage the safety notch at 1/2 way, but close, and the hammers slips out of his thumb,

THERE IS enough tension on the spring to fire the weapon

This is likely what happened