Negligent discharges injuring others or self, are of course a possibility. But, it increases from there.
I imagine the Ruger handbook for that gun expressly recommends using the safety. Can you imagine the dollar signs in a plaintiff attorney's eyes if he finds a witness who is willing to repeat what the OPer told us here? "Ladies and gentleman of the jury, I have here the Ruger owner's manual..."
Imagine what a prosecutor might tell the grand jury. "Ladies and gentleman, I am seeking a manslaughter indictment because this was more than a tragic accident. It has come to our attention that the accused deliberately never used the thumb safety on his pistol. The manufacturer of the death weapon expressly recommends using the thumb safety. This is negligence. Negligence that resulted in a death..."
William Fisher,
You might point out that real pro's know the value of the safety. And, even they recognize they're only human and make mistakes. Whatever his own personal reason for thinking the safety gets in his way, he needs to practice flicking off the safety as the first step to pulling the trigger. With a little practice, it becomes part of the firing motions, just like only putting your finger inside the trigger guard when in the very act of actually firing the gun.
He does leave his finger along the frame of the gun until he is ready to fire, right?