A few chose to be undead… 99% of the Forsaken were brainwashed and conditioned by Sylvanas and her “cult” , sorry people
A truly repentative Undead should just end her own misrable existence right and there
After all, the Undead’s existance is… basically horrible. - as you even worded it, a sin
They are trapped in their own rotting bodies, the dark magic that dragged back their soul against their will is very imperfect, so the senses are dull
It is safe to say - and correct me if I’m wrong - the undead hate their own existence
They had Sylvanas’ personality Cult to cling in to and of course each other, because they were rejected by the living and had nowhere to go (except back to their grave, obviously)
Depression and self hate but with a strong sense of community - not faction pride or loayality, they always were loyal to each other and Sylvanas
Sylvanas is gone, and they formed a council to forge themselves a new parh and future… and the question is simple: why?
Before Shadowlands they were afraid, it was uncertain what waits them (or totally certain, Darkness and Oblivion said by their Queen and their mortal teaching and faith)
But now?
Everything is okay
I mean, there is a New Arbiter and he is very kind hearted one, firmly belives, everyone deserves a chance and proper afterlife, eternal rest and peace and joy and happines, a hugg and probably puppies or kittens, depending their preference
And Kalia and the leaders were there to see his ascension and personally knew the guy, they know he is a decent, honest, soft hearted fellow who wants to help the Multiverse… wants every soul to reach its proper destination and find peace
Now… this obviously a “problem”
Why the Undead should continnue their horrible existence? Something that was forced on to them and hated every minute of it, when it is 100% they will be judged by this guy and would find their peace?
It is basically the variation of the “why Azeroth not converted in to a global Death Cult” question, but more focused on the Undead
Like to many other, sadly… thats why the dracthyr can’t use their own wings

This is a bit similar to the Lightforged Warlock “problem” - not a problem at all actually
We witness how draenei are turned into Lightforged. The Forge of Aeons reinforces a draenei’s mind, body, and soul with the power of the light after facing their inner demons and their inner turmoil
But this not means you are necesarily have to be “good”.
T’paartos faced his memories and fears, striking down physical manifestations of them. His Lightforged trial with him confronting T’paartos the Fallen, an image of his darkness who tempted him to leave the Light behind and embrace the Fel.
But a Lightforged Warlock (or Draenei) are not leaving the Light behind!
A Warlock - according the visual guide - are magical practitioners that seek to understand darker magics, among other fel-based and destructive spells. While many warlocks willingly follow the Burning Legion, there are those who work against it, using their magic to fight against evil.
Legion class preview even states, though they often pledge themselves to the service of noble causes and are not innately evil, their desire to understand darker magics and exercise unwavering command over demonic forces breeds mistrust among even their closest allies.
According the WoW game guide: In the face of demonic power, most heroes see death. Warlocks see only opportunity. Dominance is their aim, and they have found a path to it in the dark arts.
The ritual of Lightforging reinforces a draenei’s mind, body, and soul with the power of the light - so you are strong both mentally, spiritually and phisically. You have an extra layer of deffense of the Fel’s corrosive effect (you can sey, the Light already claimed you, and a jelous Force it is, guards what it have…)
During the trials you faced your inner demons, fears and weaknesses… and conquered them! Any weakness a Demon could exploit are gone! And you already - arguably - insane enough to be in a Cult. Yes. It is a cult mooooom.
The Light seeks one path and shuns all others as lies!
The Light seeks to rule, dominate, “guide” and uphold Order and Law and Justice - things that are not really defined by any universal principle, but the individual - thats why “evil” fanatics, Crimsons and so on, even one of Kel’Thuzad’s Horseman are capable to utilize it and no divine punshiment comes down to strip of their power.
What is the ultimate form of domintion and punsihment in the case of the Demons if not Demonology and the art of the Warlocks?
“By divinie right I command thu , vile creature to fight and die in the name of the Light and for me! Again… again… and again! Even Death will not save you from the wrath and the judgment of the Lightforged! By my sanctified blood I will seal our deal, our pact will be your chain and you will kill your own despicable kind because the Light… and ME wills it! OBEY!”
A Man’ari commited horrible crimes duing her probably 15K+ years of existence as a Demon.
And you know who else commited horrible crimes?
Us
The champions of Azeroth
WoW setup is… well… nothing, you are murderhobo #27773992 who would sell his grandma for a handfull of gold or would do and already done anything, including horrible things for pocket change or shiny trinkets; sold our souls and land for a handfull of glasspearls
You can’t name one, even one crime we don’t commited already at least once - okay, except the 18+ themed, “R”-worded ones (tho’ Goldshire… exist, so… )
"Because he rather liked people. It was major failing in a demon. Oh, he did his best to make their short lives miserable, because that was his job, but nothing he could think up was half as bad as the stuff they thought up themselves. They seemed to have a talent for it. It was built into the design, somehow. They were born into a world that was against them in a thousand little ways, and then devoted most of their energies to making it worse. Over the years Crowley had found it increasingly difficult to find anything demonic to do which showed up against the natural background of generalized nastiness. There had been times, over the past millennium, when he’d felt like sending a message back Below saying, Look we may as well give up right now, we might as well shut down Dis and Pandemonium and everywhere and move up here, there’s nothing we can do to them that they don’t do to themselves and they do things we’ve never even thought of, often involving electrodes. They’ve got what we lack. They’ve got imagination. And electricity, of course. One of them had written it, hadn’t he…”Hell is empty, and all the devils are here.”
―Sir Terry Pratchett, Good Omens: The Nice and Accurate Prophecies of Agnes Nutter, Witch
Similarly, no Man’ari Eredar that sit down, thinks and actively try, are capable to come up something - either solo or with an army on her side - the “Champions of Azeroth” not done already to ourselves and with each other
And we don’t even pretend we want to change!
Why should a Man’ari be any different?
Besides, we worked with ltieral Demons and most of them were quite nice, like Matron Mother Malevolence
Or remember Sal’salabim? From TBC?
Sal’salabim is a quest giver located in the World’s End Tavern in the neutral Shattrath City. He is a rebel mo’arg engineer who has taken up residence in the Sha’tari capital (yes, in the middle of Draenei-town and Naaru HQ), drinking away his sorrows
Daglop the Imp, Calydus, Avrus Illwhisper (okay he was purged of his demonic form and become a Night Elf again, but still), the Man’ari Val’zuun…
Demons have individual personalities like any other races…