There is too much confusion to make any kind of definitive statement about the shooter right now, except that it is NOT Plaz.
It may have been Sephiroth. I have not personally done the research, and I don't know him personally.
Plaz did not know Sephiroth outside of Blockland, as far as I can tell. The only potential connection Plaz has to Sephiroth is that Plaz 2.0 appears to be Sephiroth impersonating Plaz.