I can't speak for others, but I haven't talked about that scene in particular much.
I think it was badly done insofar as how it was framed. Snyder seems to have a problem setting up scenes where killing someone is the only possibility without feeling contrived. In MoS, people have pointed out how there were many ways that Superman could have diverted the lasers from the family. Fly up, block the vision, etc. Especially since snapping someones neck like that takes way more effort than most other options. If he is able to snap his neck, he should be able to prevent his head from turning. Same thing with the KGBeast. Batman shoots him, and then is able to drop the guy he's holding as a meatshield, run over to Martha before the KGBeast can do anything. You'd think he'd fire his cannister the moment Batman got off the shot. Or else you'd think he'd just shoot him in the head instead of risking Martha being burned by exploding the gas tank, in case he isn't able to make it in time. But if KGBeasts reaction time is so slow that he can't do anything in the time it takes Batman to shoot and cross the room to him to shield Martha, then it feels like a nonlethal takedown was plausible. Just run over to him and punch him out before he gets the shot off. It makes it seem when he declared his intent, Batman was looking for an excuse to kill him rather than needing to.
But in terms of what it implies for the character, yeah, I don't care about the KGBeast scene all that much. At worst, its' a drop in the bucket of all the rest of the shit he's pulled.
Yeah, pretty much. It's biggest offense is a failure to be a visual indication of Batman's character development.