I actually agree wholeheartedly with the premise that “character development” doesn’t necessarily mean “a better person”.
But if a story is going to attempt that kind of deliberate character deterioration, then maybe it should honestly acknowledge and confront all the factors which led to a particular character’s moral decay? Especially if the factors includes neglect and marginalization by other character(s) that we’re supposed to sympathize with and root for, which – surprise surprise – is something that can end up having a hugely negative effect on their intended likability if they’re never called out at any point on their neglect.
In other words? Mako has not only been extremely cold and dismissive towards Bolin (his little brother) throughout the entire Book, but he does absolutely nothing to prevent the abuse that Bolin received at the hands of the Twins. And yet not only is he never called out on his neglect and how it tangibly accelerated his brother’s moral decay, but the narrative incongruously frames him as a 100% decent and morally responsible guy.
Bolin’s stupidity (and Korra’s, and Asami’s, and Iroh II’s, and Lin and her police force’s – but those are other rants for other days) exists solely to contribute to the glorification of Mako. That’s it.
The bathtub-in-the-apartment scene – which initially seemed like it would actually be a “calling out” scene – ended up being simply another example of the narrative glorifying Mako at the expense of Bolin (whose
accidental self-electrocution scalding at the end of that scene* is framed as “karma” for his stupidity).
tl;dr - Nice try to justify the shit writing and character derailment, but no cigar.