As far as Negan just being a pragmatist (and I don't know the character from the comics) I'd have to disagree that he's just someone doing what is necessary for survival and assert that he's evil.
A pragmatist would kill enemies quickly and efficiently and only as a last resort if the opportunities for mutually beneficial alliances could not be attained. Negan is an obvious sadist who relishes in the pain and torture of others.
Obviously I dunno what the next handful of episodes will be like, so it's entirely possible that they'll make him more of a sadist than he is in the comics. But I don't think they've closed this off yet, for a couple of reasons. First, because the horror of what he's done is obviously meant to destroy any inkling of rebellion they may have, so any messed up thing he does could be framed as just a means to that end. And second, because I think a lot of it is indifference more than revelry, though in the face of such violence, they start to look pretty similar.
Or, indeed, they'll mess it up horribly by just turning him into a slightly more jovial Big Bad than the last two or three. Wouldn't be the first time they did something wrong.
Even in such a fantastic scenario, civilization could easily rebuild itself. There would just be very different protocols for how the terminally ill are treated and how the dead are disposed of. Hospitals, morgues and funeral homes would be equipped with cages, restraints and those bolt guns that are used in slaughter houses to kill cows - to distribute a bolt to the brain of every newly deceased person before cremation. One thing about humanity is they have an uncanny ability to rebound from wars, devastation, disasters and disease.
Yeah, there should be a lot more of this. I'd be pretty interested in a version of this show that's much more political/philosophical, with survivors debating whether or not they should try to rebuild the world as it was, or start over completely, or something sort of in-between. You can see a lot of people saying "whatever we do, it shouldn't be the same." Political idealists of one stripe or another would see it as an opportunity to ("now we can abolish property!"/"now we can have our libertarian utopia!"). Even if they come down in the middle, you're right, they'd have to change the way they respond (both legally and emotionally) to death, murder, prison, etc. You wouldn't be able to waste resources on prisoners, so the death penalty would have to be much more common.
Battlestar Galactica did this in an interesting way: by banning abortion. Humanity's numbers were dwindling and the government (such as it was) decided they needed everyone they could get. It was an interesting spin because it came at the issue from a direction totally foreign to the ones people approach it from in real life. And that's what would happen is this were real: lots of present day political issues would be considered from totally new angles.
Why don't they do all this? I'm pretty sure it's because the overwhelming majority of viewers watch because they like to be grossed out by bodies getting ripped or smashed apart. Anecdotally, I saw lots of people, during the first couple of seasons, complain when there wasn't much gore. And when there was something particularly gross, that was the thing they singled out. They just want crazy, brutal sh*t to happen.