Look, Bolvar got engulfed by Alexstrasza's lifebringing flame breath, which doesn't have much to do with hell, does it? It's a "redeeming flame" more akin to holy or nature, if anything. Just because DK's are often considered damned souls, does not mean that their future has ANYTHING to do with "Hellfire." I understand this is mostly a colloquial term, but it is simply incorrect.
Hellfire: https://www.wowhead.com/spell=87481/hellfire
For a long time now, the Burning Legion, Sargeras and the Twisting Nether have been the WoW equivalent of demons, Satan and Hell. Demonology warlocks use Hellfire. It's a demon thing, it's fel magic.
Please, stop with the Hellfire Death Knights.