Southern

I'm not really A Southerner. I was born and raised in the Deep South but my parents were from elsewhere. 

I don't think saying "why, bless your heart" or calling someone sweetie is really intended to be bitchy cloaked insults.

I've just written a piece where I said:

...please realize I'm trying to help YOU achieve the goals YOU want and wherever you go, there you are, sweetie. (That's Southern for RETARD in this case.)

Most of the YouTube videos or real life anecdotes about those expressions involve trying to explain stuff to someone new to the South.

I think Southerners are probably very often doing what I'm trying to do above and be warm and friendly to someone doing something stupid in hopes of catching more flies with honey than vinegar. Because my impulse is to try to express affection for my audience in hopes they might listen instead of hating on me and accusing me of victim blaming.

But long experience tells me most folks don't want instructions on how to stop cutting their own throat and a ridiculous number of people are eager to imagine I LOVE them and I'm their one true love or new best friend for life, by which they mean their bitch who will make their life better while they piss all over me.

So I added the parenthetical information to say "Let's not, mmmkay?"

I suspect what happens is someone takes pity on you and instead of saying "God, you're an idiot. Your neighbors are doing that bullshit intentionally. I know because I know these assholes." they say "Why, bless your heart." about your frustrations and then make a polite suggestion for how you could handle it differently because they don't know you and have no plans to let you tell your neighbor what you really think of them and have that come back to bite them in the ass.

And about the fifth time you express similar frustrations and you've never taken any of their advice, they are tired of hearing it and done trying to politely clue you, so the exact same words become tea and sympathy in hopes of discouraging you from continuing to flap your jaws and waste their time.

Kind of like that song Everybody Wants You where the same line means something different later in the song.

My ex is kind of a selfish asshole, what frequently gets called a narcissist or antisocial misandrist on the internet. Our oldest son is like his father and likes to be left the hell alone be people. 

And that boy spent his childhood watching his dad be ugly to people in hopes of getting some peace and quiet and it coming back to bite him and watching his mother be polite and diplomatic and getting actually left the hell alone by assholes most of the time. He decided diplomacy is the way to go if you like your peace and quiet.

So I'm not really Southern, but I strongly suspect that's the progression behind new people hearing "Why, bless your heart" and then one day getting told "That means you're an idiot."

It didn't mean that exactly initially. It meant "Oh, God, THIS is stupid. Let me try to clue him diplomatically without cutting my throat."

It came to mean YOU are an idiot because of your proven track record of refusing to get a fucking CLUE.

It's nuanced and contextual and probably most online "humor" misses that fact.