To kill WoW you need to have a budget equalling the current value of what it took to develop the game and all of it's expansions at a minumum, and probably more. You need to basically create more content than the competition has at the time you release to keep people playing. Too many games fall into the trap of having decent gameplay, but not enough stuff to do, which is why people quit and go back to other games. The justification that "we have as much content as WoW did when it launched" doesn't work because you aren't competing with WoW (or other games) when they launched, but what they are right now. To get people to pay to grind your endgame, there needs to be decent endgame content, and it needs to be entertaining to get there. Plus you need to have stuff to keep players who aren't interested in endgame type behavior (raids, PVP) entertained and playing.
The Halo MMORPG would not have killed WoW simply because had enough resources been invested in it, it would not have been cancelled since too much would have been riding on it.
That said I'm not surprised it got killed, the very idea of a "Halo MMORPG" is very similar to what Richard Garriot tried to do with "Tabula Rasa". Basically a military themed shooter MMORPG based around fighting an alien invasion. That game failed epically, basically showing (I believe) that a format/world based around shooter type conventions isn't going to work for long term, persistant play. Looking at what happened to a similar game, combined with things like "Old Republic" that have been in development for a while, yeah... I can see why they dropped the axe on this as an epic fail waiting to happen.