Supposedly, the nature of Edolas prevents magic from being stored in the user's body or wielded. That means all Zeref needs to do to avoid killing everyone is to find a way to get to Edolas.
Even if the immortality remains (which is unlikely in my opinion) the death aura is a form of uncontrollable black magic (http://fairytail.wikia.com/wiki/Ankhseram_Black_Magic)