Mishandling a weapon makes my blood boil. I was at Gander Mountain looking at guns. They guy took the gun out, removed the trigger lock, pulled the mag and opened the slide. Then stood there talking to me pulling the trigger pointing it all around. He then set it on the counter with the barrel pointing right at me. I reached forward and turned the gun away from me, and anyone else. I then said I'm very anal about gun safety and I get very uncomfortable when I see them handled in a unsafe manor. He just didn't seem to get it.
One mistake, and you, or someone else is dead. I'm not willing to live knowing my mistake cost someone else their life. It's not a game. Safe habits are the key. Impartial observation helps. Even setting up a camera to video your time on the range, then review it at home. You may be surprised what you will see. 
I agree with you - bad handling makes me boil. That's one reason I tend to avoid gun shows. For the most part, I find myself very uncomfortable inside a gun show with all the bad handling etc. that goes on by not only the clowns and wannabees, but experienced folks who SHOULD know better but seem to forget basic gun handling in that forum. It's actually frightening in my opinion - like being at a ski resort in the base area with too many non-experienced skiers trying to shoulder their skis and swinging them around haphazardly and almost hitting you in the head - I felt safer playing hockey! BUT these are not skis and are therefore much, much more serious - not a game like you said.
In gun stores or at gun counters in stores like Cabelas or Gander I've gotten a clerk or two upset when they've pulled out a gun, checked it and then handed it to me closed and then I checked it again myself. One even snottily said "I've checked it, don't you trust me or do you think I'm too dumb to determine it's unloaded?!" Well, as a matter of fact, no I wouldn't have thought your were dumb UNTIL you said that and NO, I don't trust you. I'll check it myself thank you! This was at a respectable gun store with a guy who probably worked there 20 years.
Familiarity breeds a certain arrogance and with that a certain over confidence that leads to simple mistakes. It's like the stories you hear about some experienced electrician who dies of electrocution after they make a rookie mistake any apprentice would avoid. Again, not a game and a simple "oops, sorry, my bad" won't cover that errant bullet that just hit someone - you can't take that back!
Another time I'll share some honesty about my own screw up - hole in the ceiling - nobody hurt, no cops, no damage that couldn't be fixed - poor dog scared but I will say as a gun owner/shooter - BEST thing that could have happened (I mean in the way that it happened - nobody hurt, no legal issues and scared the holy crap out of myself) and it was a one and only time mistake. Drove the safety point home to me forever. Mentioned it to an old timer/cop I shot with at the time and he agreed that "nobody hurt and a good thing - slaps you the ____ out of being complacent!" Thanks to the topic starter
manheiritis for the honesty and starting a good and important thread! Cheers. D.