Frame challenge: these necromancers are evil.
I'm not kidding. Bear with me:
These necromancers do not sound like good people to me.
It appears they appointed themselves absolute rulers over the local populace, who are all criminals -- or so said the prior political order, which, as you say, just collapsed.
It sounds to me like the residents of this jurisdiction just rid themselves of some bad rulers as well as some or all of the political system those rulers had imposed.
You describe it as "chaos," which is easy enough to believe. Neither the French nor American Revolutions were tidy operations, and each was followed by years of political turmoil (to say nothing of social or economic waves). Eventually, both states stabilized; my French history is dusty, but I know it took the Americans eleven years to get from that first angry Declaration in 1776 to the federal Constitution in 1787.
But I guess these necromancers decided that time was up! The locals had failed to establish a new political system quickly enough that satisfied this group of powerful elites, so they just used their supernatural might to install themselves as the supreme political power.
Did the locals care what these necromancers might think of their journey toward self-government? Did the locals even know the necromancers existed before they seized power for themselves?
Of course, it's not described that way. They didn't seize power "for" themselves. It was "a necessity." I ask: who needed this? They say their "only" demand is to be left alone to do research. How reasonable is that request?
In a medieval setting, where many households must daily fetch water in buckets from nearby streams, hunt and kill their own food, and defend themselves from scofflaws in the absence of any competent policing, "being left alone" is truly a fantasy reserved exclusively for the landed nobility, who have servants for the work of maintaining a household. Quick googling suggests something like 5% of the population would be so fortunate (numbers vary by country, but 10% seems like the high end).
A handful of pampered, super-powerful people were in need of a new home. Why? Because they just got expelled from the company of their peers' for refusing to heed any of the countless warnings, admonitions, nota benes, cautionary tales, professional rules of conduct, written laws, stern yellings-at, and, finally,
desperate entreaties from loved ones, literally begging them to stop playing with death magic.
But no, not these guys, they know better than literally everyone else. So they burned that bridge, and then I guess... roamed the countryside for a while? I imagine they probably helped themselves to some midnight exhumations ("your late father had a very particular kind of body hair that is not well understood by others in my field, which is why I had to defile his grave and interfere with his eternal slumber, and also why his reanimated corpse can't learn to make a decent MLT to save its life!").
One day they stumble upon a pocket of civilization large enough to support a class of parasitic elites, and by happy coincidence there was an opening in the penthouse apartment! What's more, the power vacuum also rendered the place defenseless: nobody in the crow's nest watching for roaming bands of supernatural predators, nobody at the helm to recognize the threat or order defensive preparations, nobody even to organize a formal parlay in hopes of deflecting the predators with a story about plague and an offering of deceptively meager trade-goods.
Possibly we have slightly different ideas of what constitutes "bad guys," but it seems like the central challenge is that the necromancers have no legitimacy as rulers, and are only in a position to impose their notions of order because they happen to be magic users.
Given that, I'd recommend:
- show how bad things were before the necromancers took power
Before they meet the necromancers or even hear stories about them, have the PCs encounter some victims of the post-collapse turmoil. Make them viscerally sympathetic: dad was a professional under the old order, then joined some kind of last-ditch citizens' brigade to prop up the collapsing government, which is how he got injured. The old order collapsed anyway. His injury prevents him from resuming his old profession, so even though the law of the tower has mostly restored the economy, he and his family are trapped in poverty. If you really want to pour it on, have this family host the PCs for dinner.
- show how bad things are in nearby areas outside the necromancer's control
Ideally you can do this while the PCs are traveling through that territory on their way to necrotown. Making it a total wasteland robs you of storytelling opportunities, and anyway you're trying to show that the reason the necromancers are good is that they keep everybody in line all the time. So, if your definition of "good government" is literally just "very tough on crime," have the surrounding region be populated by people who are all scheming thieves. There are lots of cunning traps that can be set for unwary travelers; everybody and their mom would have a few of these traps set up in their own homes just in case some really easy mark rolls off the turnip truck. "Traps" doesn't mean boobytraps, more like... pre-arranged scams. (Although you could go with some kind of Appalachian horror, where locals sometimes capture travelers and keep them prisoner in the basement.)
Think about it this way: if your cunning neighbor leads some simple-looking strangers into his own home, he's probably lining them up for some kind of con. So it stands to reason he'd be grateful for any help you might volunteer in making that scam work, as long as you really do help instead of spooking the target.
He might let you wet your beak if you can help him separate these rubes from their wallets.
- mark the boundary between necroland and everything else really clearly
However large is the region over which the necromancers have power, put something physical there that serves as a notice to people entering that this is the realm where the law of the tower holds sway. Not necessarily heads on spikes, but considering that the necromancers' claim to legitimacy is that they are "tough on crime," it seems appropriate for the boundary marking to somehow be about the edge of the legal jurisdiction as such. The town in Unforgiven was ruled by an evil sheriff who used this sign to alert travelers that they were entering his domain. It doesn't say "Little Bill Is King", it says "No firearms in Big Whiskey." But the point is the same.
The reason for marking the boundary is two-fold:
- you're about to show them how good things are in necrotown, so make sure they know it's necrotown
- the laws really do change there, so you and the players need to always be conscious of which side of the border events happen on, so that players and NPCs can act accordingly
- show how good things are in necrotown
It doesn't actually have to be awesome, it just has to be better than the other areas in ways that are clearly directly related to the prevalence of crime.
- finally, have them actually meet the necromancers
Hopefully now the players will be receptive to regular pleasantries and the polite suggestion that perhaps other magic users are simply mistaken to ban necromancy. This is when the villain would naturally make a pitch for their vision of order: they think they've just met kindred spirits, they are impressed with their own accomplishments, and they practically can't help themselves just going over all the primary and secondary advantages of this way of life. They'll be like enthusiastic kids who can't wait to introduce you to each of their toys one at a time, to demonstrate and explain each of its cool features, and then to move onto the next one literally without taking a breath between.
These necromancers have a myopic view of things, so two themes will emerge in all their dealings with the players:
- all their stories and anecdotes about how good things are will be told from the perspective of pampered elites who each get to devote all of every day to their favorite hobbies (because they have put the state on autopilot)
- they will talk about the local population as means only rather than ends, as a problem to be managed or solved rather than as individuals with inherent worth, as cattle to be herded rather than peers whose preferences are no less legitimate
- avoid letting the players directly witness the necromancers using their power
By "power" I mean both their magical necromancy as well as their official powers as dictators.
If you're worried that the players are not likely to sympathize with necromancers because death magic sounds bad, then you probably want to avoid forcing them to confront the gory details. It will be easier for them to either ignore the question or fool themselves into thinking it's "totes fine" if they can do some from a place of comfortable ignorance.
And if you're worried that the players will not want to ally themselves with magical tyrants whose grip on power is literally cold and dead, don't let the players see the sausage get made. Don't let them see the necromancers pronounce new laws (especially making things illegal), or sentence anyone for a crime, or handle any political or governmental challenge, because people do not normally sympathize with anyone who gets to dictate to others.