Dark Knight job vs Death Knight (WoW)

[Nahara]

what reasons they will have for joining the war effort

If I were to take a wild stab in the dark, considering we're going to Ishgard and Isghard has been closed to us since forever, I would say that Dark Knights may have always been a thing in Ishgard and we're just now getting introduced to it because we actually get to go there. Further, I'd say that Dark Knights might be a more secretive order of knights, as opposed to the publicly known Temple Knights and the Dragoons (maybe acting as the Holy See's black-ops group or something like that).

As for how the Dark Knight compares to the Death Knight... it seems not many people here played WoW up to the introduction of Death Knights, but I can say in almost* no uncertain terms that you were just flat out evil. Like straight up, unabashedly evil. You worked for the main villain of the expansion, you murdered anyone and everyone that got in your way in cold blood, probably resurrected them into undead servitude, and maybe even exploded their remains in a violent spray of gore when they outlived their usefulness (though, admittedly, Corpse Explosion was pretty hilarious). Breaking away from the main villain and joining one of the factions had more to do with the fact that he sent you on a suicide mission and you want to exact revenge on him for that, rather than any kind of reformation or desire to do good (not that either faction was very altruistic to begin with).

Dark Knights, as pointed out already, are not evil (and also still living). They may have dark powers, but they are wielded by good people (eg. us) and used for good purposes (eg. fighting evil).

*: I say almost, because the people you kill in the Death Knight starting area were the Scarlet Crusade who are also evil and you normally just murder them whenever you come across them anyway.

[Alenore]

snip

Not exactly. At the beginning of the story, the Death Knight was simply raised by the Lich King (or one of his necromancer), and is pretty much a slave. He has no choice but to obey him, except for a very few.
Thing is, that min control is broken during the story and from that point, while you pretty much want to pwn the Lich King for revenge, you also get back the feelings and stuff. For instance, Thassarian still cared for his sister, tried to waste as few lives as possible, ...