actually it wasnt Seph, it was Jenova, controlling a seph clone, or sumthin
Yeh, there's that whole controversy about whether it is Sephiroth's will that creates the "sephiroth" that kills everybody or if Sephiroth is truly dead and it is Jenova who is using his image to kill people. I dunt really know which one is true or not, but if push comes to shove, I'd have to say it's all Sephy.