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From what I’ve read, evil-aligned celestials are an extreme rarity, but evil-aligned deities exist. I’ve even heard that if a celestial turns evil, they stop being a celestial. Celestials supposedly serve deities, but it’s never been specified what type of deity they serve as far as I know.
It is possible for celestials, even the most powerful ones, to be tricked by evil deities. They might become pawns, initially unaware that they are being played with. They might eventually get corrupted and fall. A famous example is Malkizid, a solar who was seduced into betraying Corellon by Lolth, and was later cast into the Nine Hells. Another example (from 3.5e) of a celestial actively serving an evil power was Thah Rhalar, a hound archon minion of Set.
If you want some inspiration about how to modify celestials to serve evil deities, you might have a look at the description of angels serving the God-Pharaoh, who is actually the malevolent dragon Planeswalker Nicol Bolas, in the Plane Shift: Amonkhet write-up available as a free pdf from WotC. While not legal in Organized Play events, it is still some kind of "official" D&D-5e-compatible material.
Zariel (Mordenkainen's Tome of Foes, page 180) was a celestial (solar), but she now serves Asmodeus as the Archduchess of Avernus. However, her fall seems to have turned her in to a Lawful Evil Archdevil. I don't know whether she could have stayed a celestial, or whether she could have kept a Good alignment.
While these three examples below are setting specific, they are still evidence that Evil Aligned Celestials can and do exist in D&D, generally most Celestials are of any Good Alignement, but their are some that are Neutral or Unaligned, and with those three being evil I see no reason for their not to be other Evil Celestials that can or do exist. Based from those three alone Evil Celestials fall generally into Lawful, still fulfilling the typical Lawful alignment of angels that they represent, however other Neutral or Chaotic Good Celestials exist, so a Neutral or Chaotic Evil Celestial is just as likely, otherwise a DM will have to devise their own stats for such Celestials.
Ashen Rider, Lawful Evil (Mythic Odysseys of Theros) - The fall of the ancient archon empires left some archons bitter and resentful. Seeking to avoid the spread of human civilization, these archons made their way to the Underworld. The horrors of the place broke their minds, bodies, and spirits and twisted them into the terrifying archons known as ashen riders. When they ride forth upon the mortal world, terrified mortals make offerings in a desperate attempt to appease them, but the ashen riders aren't merciful, and they delight in reducing the paragons of the mortal world to ash.
Deathpact Angel, Lawful Evil (Guildmaster's Guide to Ravnica) - Deathpact angels dwell in the grandest of Orzhov cathedrals, where they surround themselves with wealth and wretched vassals that are utterly in their thrall.
Radiant Idol, Lawful Evil (Eberron: Rising from the Last War) - A radiant idol was an angel that was banished from the celestial realm of Syrania and cast down to the Material Plane. One sin led to their fall: the desire to be worshiped by mortals. Now in the mortal realm, most radiant idols gather cults of devoted followers.
Good aligned outsiders, per the Monster Manual, are Celestials. Evil aligned outsiders are Fiends. The Subtypes differ for each, so making a subtype of another kind would work. An Outsider (Eladrin, Demon) would have the evil alignment but the Eladrin and Demon subtypes. It works within the rules. An evil deity can still do good deeds, but usually for evil purposes. A Celestial would promote the good deeds, but not the evil end, and could unwittingly be used.
