Framing wise...I thought Mollie was rather dumb when it came to her husband. Dumb and just unexplored how she never thought he was a part of anything, especially when she trusted him with the medicine in the second half.
Your perspective is much too macro here. Love clearly played a huge role in why she couldn't see what Ernest was doing. If Scorsese had written her the way you would rather, she wouldn't have even seemed human. How many husbands or wives who get scammed/murdered by their spouse are able to see it coming? There have been men throughout history who got away with
serial scams of multiple wives and it's not like many of them were sociopathic geniuses.
I don't remember everything I read from reviews, but I saw that Ernest wanted Molli there the night he blew the house up with Mollies sister or whoever that was.
I seem to remember him not wanting her to visit her sister. Can't remember, not sure what the point is though.
Going further, I thought there was no explanation or analysis or even acknowledgement that Ernest was a piece of garbage and Scorsese instead just played it off like he was an idiot.
My assumption is that Scorsese did not have a lot detail to work with in regards to character development and instead left it that way, rather than creating something from nothing.
Definitely disagree, Ernest was very clearly depicted as naive/stupid yes, but also more importantly very greedy and a huge coward. This is why even in the end he couldn't admit he poisoned his wife. He usually pawned off the dirty jobs to others. He was drafted but somehow managed to avoid combat as a cook. He got paddled by his old ass uncle when he could've easily resisted lmao. He almost didn't even testify against his uncle until his daughter died of whooping cough and he assumed his uncle did it.
In fact I think one of the film's best qualities is the way King is slowly revealed to be unhinged in how malicious he is, and by contrast Ernest is just a spineless fuck who actually wants to do the right thing but never will.