Then neither would a Necromancer's healing spec. It would be game mechanics. And it would still be thematic to themselves.
I mean look at Priest lore. We have Gnome Priests who are 'Surgeons', Tauren Druids who are 'Seers worshipping the Sun' and Draenei who are followers of the Naaru. Where is the lore regarding the use of Old God Void magic for each of these identities? Shadow spec more or less exists outside of canon. Specs and their abilities need not be canonized into lore. Their names and themes are just flavour text to help integrate them into the Warcraft setting. There is no implication that your character would ever be subject to madness through using Shadow magic, whether you're a Shadow Priest or a Void Elf.
The entire idea of 'Necromancer can only be evil' only applies to NPCs. If we're talking about a player character, like I said, we all exist outside of that bubble. These characters exist because they aren't bound to lore. Your DK may have a backstory connection that explains how they serve in relation to the Horde and Alliance, but by all means your character and what he/she does is completely up to you to define. You can be a pacifist DK who only levels through Herbalism if you choose to be; that doesn't define your character as being good or evil.
And to date, you haven't given me any examples of good Warlocks in the lore. You've managed to excuse them pretty easily, despite there being zero good Warlocks in the lore. Would you take time to address this in your argument?