Yeah, this ending is anti climactic and I don't like it. However I knew zeref would be defeated like this. It was predictable hundreds of chapters ago.
Zeref is cursed with the curse of contradictions, making his entire existence a contradiction. He should be mortal, but he's immortal. The more he loves life, the more he takes it away. He wants his little brother to kill him, yet he wants to kill him.
Even his mind is contradicting itself.
So then how would he be killed? I didn't know exactly what shape it would take, but as soon as he took fairy heart it became obvious. When natsu attacked him he regenerated, showing that he has become immortal. However at the same time that was the first time in the entire series he was shown to have taken damage.
Before when natsu used igneel's power it did pretty much nothing to zeref, this time it obliterated his entire body.
He had actually become immortal. So with the curse of contradictions, he who should be immortal, is mortal.
At least that's how I would try and explain this ending... doesn't make it any less anti-climactic though. Especially for acnologia ^^