Azuth from FR might be a candidate. He is not a god of magic, but of wizards, and is mostly interested in the structured study of magic. From wikipedia:
The clerics of Azuth teach wisdom and restraint. This means that Azuth is often not the favoured god of sorcerers with their fiery, instinctual use of magic, and bards with their somewhat fanciful approach to it.
From the 3e FR Campaign Setting, the first sentence for Azuthian dogma:
Reason is the best way to approach magic, and magic can be examined and reduced to its component parts through study and meditation.
Likewise, quoting from the Sword Coast Adventurer's Guide (SCAG):
Azuth is god of a wizard's long hours of study, exacting standards of movement and speech, and cramped, ink-stained fingers.
Your cult might have decided that pact magic is an affront to the real teachings of Azuth, essentially a cheat to gain power without real work, swapping sweat with stolen power; without a proper understanding of what is going on. Warlocks could be seen as fools who are endangering the society with their broken understanding. Moreover, since the common folk cannot necessarily distinguish between actual wizards and warlocks, pact magic users will eventually cause serious paranoia within the society against reputable practitioners of magic, ie. wizards, as well.
Also according to SCAG, there are few dedicated centers of Azuth worship, and this fact might play well in your game. While there will be clergy who see this cult as heretics, they will be spread out and might not be able to mount a concerted argument to suppress the cult's "distorted" view of Azuthian principles. (An additional piece of lore information from 2e Faiths and Avatars: 45% of the titled clergy within the church hierarchy used to be wizards. Given that the 5e pantheon is structured in almost the same way as the 2e&3e versions, I don't see any reason if you wanted to keep this ratio also in 5e.)
Finally, Azuth is LN, so the cultists could see themselves as principled saviours who are willing to make the hard decisions others are unable or unwilling to do. What they are doing is to maintain order for the greater good of the society. When witches/warlocks are captured, the cultists might offer them a chance to renounce their pacts, atone for the bad deeds they did under the taint of otherworldly patrons, and start a new life learning magic the proper way. You could even imagine multiclassed warlock/wizards who joined the cult this way and having been converted, they could be even more zealous than the others in the cult.
PS: Following a comment by KorvinStarmast, it might be worth mentioning that within the 5e rules, you might have one or more Arcana clerics of Azuth playing important roles in this cult, either openly or covertly. I can add that such clerics could even be of LE alignment; not only 5e allows it, but also as far back as the 2e, Azuthian church accepted LE clergy according to Faiths & Avatars.
The question has a dnd-5e tag, but in the interest of providing an avenue for the rather interesting concept, here is a lore answer from earlier editions. Most of what I write in the next three paragraphs come from an article named "The Cult of Ao: Who Watches Those who Watch over You?" by Steven E. Schend, published in Polyhedron magazine issue 94.
After the Time of Troubles (ToT), a cult was formed around the worship of the overdeity Ao. While the cult lost most of its followers once they realized that Ao did not grant any spells, some stayed. Those who stayed formulated tenets that reflect what Ao had declared during the ToT. In summary, as far as I understand, these tenets roughly state that Ao wants a balance, all the deities need to tend to their portfolios and to the needs of the worshippers in order to maintain that balance, the duties of each deity and his/her followers are equally important.
Thus the cult approaches all deities from the same perspective. If the worship of one deity is too strong in a given region, they support the opposing deities. They watch churches to see if the priests are really doing what they should be doing to further the portfolio of their deities or simply trying to amass only personal gains. For example they sink a ship full of opulent cargo meant to be used to furnish Sune's temple in Waterdeep, and that cargo ends up in the shrines of mermen deities.
Members of the cult were not faithless, they had their patron deities. For example, the leader of the cult at that time (circa late 1360s) was a wizard and a worshipper of Oghma. Some were even priests. They were usually very devout and embraced a conservative interpretation of their gods' teachings, as keeping the balance meant the clergy of each deity had the duty to act precisely in the way that advances the portfolio carried by that deity.
A lot has passed since the publication of the above 2e material, but remember that the people of Faerun have encountered Ao again very recently, as he announced the end of the Era of Upheaval, concluding the Second Sundering. Given that the 5e FR has the deities not interfering directly, the fate of the world is left to the hands of the mortals, and the cult might have rekindled. The divine order of FR needs better informed mortals now more than ever. Your character could be a devout cleric of Oghma or Savras, as you mentioned in your question. Other worthy candidates, who support the Knowledge domain and could support cleric members of the cult of Ao, are Deneir, and Jergal:
Deneir: a power who is working with Metatext, a work that records the whole of reality. He is also closely allied with Oghma. Your character might be one who understands the order dictated by Ao needs to be searched in every corner of the world, collected in writing and brought to the masses, who need to understand that the deities play important roles in that order and need to be worshipped.
Jergal: a power that used to hold Kelemvor's portfolio in the ancient times. He would be very familiar with the faithless as he is the scribe that keeps the records of the dead. Warning: His uncaring ethos seeing life as a temporary step in the eternity of death requires careful consideration.
Best Answer
One would think any good (and frankly even many Neutral) gods would have a place for opposition of black magic and foul sorcery. That's kinda like saying "who doesn't like murderers?" "Anyone but Norgorberites I guess?" is the answer. Even CN churches won't like it if there's enslaving/domination going on from the baddies.
Good type gods that are the most martial are Iomedae and Sarenrae. Gods with more of a craftsman aspect that might be Black Powder friendly are Torag and Abadar.
You'd do better to include what AP you're playing in and what kind of character you'd like to be like beyond the class details to get a more helpful answer. What part of Golarion? What kind of bad guys? If the answer is "necromancers" then suddenly Pharasma jumps up pretty highly in the choices, for example.
If you'd like a "Conan the savage who doesn't like arcane magic" feel, then the Kellid race and Gorum as a deity makes a lot of sense. Kellids from Numeria get some gun contact and would work very well for Reign of Winter (they hate White Witches), Iron Gods (from the area), and Wrath of the Righteous (from the area).