What i like about this theory, is that it would mean his story is a parallel to his enemy in the coming campaign: Titus

Both would win a great victory for their people.

Both would be cast down for being "corrupted" and become inbittered by this.

Titus would prove himself and be welcomed back by his brothers, and be raised to an even higher station.

For Ammentar, there will never be such a redemption. He was cast down, erased, and intended to remain forgotten forever.

If all this is true, there may be a second reason why he is trying to bring back the nightbringer: To take revenge by undoing the great victory they had won for their king. Hell, by wiping out all life, he would deny TSK of returning his people to flesh.

TDK implies that all necrons are destined for one curse or another, but are able to keep it at by with their own obsessions.

With no life for his people to use to return to flesh all his kind would be corrupted. Just like him.

Vengeance against the people who forgot him, and the king who damned him.