Seung expressed disappointment for Christianity in his note before the bloodbath.
“He also expressed disappointment in his own religion, and made several references to Christianity, the official said.”
This is extrememly obscure, and poorly written. Yes, it could mean he is disappointed in Christianity, but what it says is, He is disappointed in his OWN religion? and then makes references to Christianity.
In any case, my personal guess is the guy was inhabiting his own fantasy world. Who knows what he thought his religion was.
From the link you provided:
He also expressed disappointment in his own religion, and made several references to Christianity, the official said.
It doesn't say that he was Christian, or that he was expressing disappointment in Christianity.