Did you have forget how Gray never obsessed about Deliora once it was sealed? Lyon on the other hand wanted to unmelt it so he can defeat himself. So if END was already sealed why would Gray obsessed about it? The answer is Gray is CORRUPTED.

How it’s acceptable?
You answered your own question with the bold:

I bet Momento Mori has fucked him up since Natsu even mentioned about it.
Gray is not acting within his character…..

Or maybe it’s just a great actor who deserves an Oscar