Words have meanings, connotations and origins. In Spanish, people use a pejorative synonym of way to indicate cowardice, to give a different example. The word itself may be used without malice (hell, if done it quite a bit in the past and it's something that I try to avoid now), but there is still much more behind the curtains.
I'd say, yeah, again, bitch would probably be something to be careful about. The problem is the continuity and naturalization of those words, especially when there are alternatives that exist and lack those kind of connotations. If they used a neutral insult, you wouldn't have noticed. But words are perpetuated by its use, and as such its us, the speakers, to decide what are kept and which aren't.
Writers must pay attention, think of what they're writing and not just follow what everyone else says. Changing cunt for a synonym takes a few seconds, makes no difference, and keeps a more consistent and inclusive power fantasy for all parts involved.
Again, if the use of sexist language and language with sexist connotations is something that's actually important, or necessary to the character, then go ahead. Awful people are allowed and should exist in narratives. Hell, Donald Draper (Mad Men, TV series) its one of my favorite characters in media and he's definitely not a good person. But that character, flawed as he was, made sense and those more unsettling parts of him were important to the story the series was trying to tell.
A writer has a duty to the language he uses, because the use he gives to it can influence how it will be shaped and used in the future. Everyone is free to write however they please, but they're equally free to critizise something they don't like.
I'd argue son of a bitch is more related to being low class, and associated with crime (most of them would end up in low class orphanages), so I'd say the connotation is different, but I may be wrong and I can stand corrected.
