Are we sure that Cowl and Kumori aren't Nemfected?
No, I am just skeptical of attributing everything weird that is happening to Nemesis.
From various hints in the books and WOJs there seems to be some kind of big cosmic thing coming up that is known to most of the major powers (now, with PT, presumably this has something to do with the Starborn cycle of 666 years).
So there is reason to expect a lot of unusual activity, even without Nemesis.
Remember that the main sign of Nemfection is people acting against their basic natures -- Kumori is a necromancer who is trying to use it for good. That's pretty much the opposite of the standard nature of a necromancer.
Do we know that? All the necromancers we've seen have been connected to Kemmler in some way, and he was super-evil. So arguably these aren't "independent" instances of necromancers going evil, they are all derived from one source.
And the Laws of Magic mean that any necromancer is going to be hunted by the Council, which will drive them to more desperate measures.
Harry doesn't seem corrupted by using necromancy to animate Sue in DB; we don't see him tempted to use necromancy again, the way we see him tempted to use magic to kill a few times in the early books.
Sure, maybe that didn't *technically* violate the Laws since Sue isn't human, but Harry also says that necromancy uses a fundamentally different kind of energy than regular "life" magic. So if that energy was inherently corrupting, we should have seen that, IMO.
So I think there is some (though fairly weak) evidence that necromantic energy isn't inherently, per-se evil.
(And there could be a very good reason not to use necromancy to help people the way she does -- remember that Aurora was certain that she was doing the right thing, too. The DV is a world with a real, empirically-verifiable afterlife, so "ending death" might not be a pure good.
Oh, absolutely. I think that it's definitely misguided and they are willing to do evil things to that end, so they are villains. But people can be evil without being corrupted by black magic or Nemesis.
My view is that only the 1st-4th Laws are directly corrupting in the usual sense, the 5th-7th Laws are more "don't mess with reality" sort of things.
Though Harry's experience redirecting the entropy curse in BR does kind of suggest that Outsider-powered magic might be inherently corrupting... but then that might just be him reacting against its "anti-reality" nature.