Why can't Dark knights be epic like this?

[Malzian]

Probably infatuated with the large sword and void powers to raise the dead to control without regard to the story (like the Grineer wanting to cause mass genocide - you're not even evil as Nekros lol).

Eh, Nekros isn't really all that good like that. The only real reason to use Nekros or Nekros Prime is to keep your desecrate skill running for extra loots. All the rest of its abilities are a waste, honestly. Thematically I understand, but Nekros doesn't even need to use that sword, nor does any frame specially come with any weapon attached (aside from Exaulted weapons, but that's different).