User blog comment:Martialmaniac/Illumi's necromancy/@comment-25814498-20151020211607

If the Ten Dons are really that careful about keeping their whereabouts a secret, it may just be that the only people that knew their location was everyone present at the mansion. But since everyone at the mansion is now dead, noone knows where the hell they are, so Illumi can continue to manipulate them as Needlemen without risk of their corpses ever being found.

While it would be cool is Illumi could control dead bodies, I don't think it's actually a thing. What Killua said was that as soon as his Needles enter the brain his Nen traumatizes them for life, turning them into vegetables, but in the 10 Dons case, they had more than a dozen needles sticking out of their brains! Even without any Nen attatched to them, that'd be lethel to us puny mortals, and so they were as good as dead either way.