Sephiroth didnt' go out and kill Aeris. He stayed away in his big hole and manipulated people Hojo had injected with Jenova Cells.

So...it wasn't even his doing. He just got handed people he can control. Not even manipulate which takes brains. he just has power over their screwed up minds.

And if Kefka wanted a city destroyed, he shot a beam of light at it and goodbye.