Sephiroth's got the guts; after all, he nearly destroyed the world. Not just metaphorically, like Kefka managed; his plan would have wiped out all life on earth. So I think it's safe to assume he'd have no problem with poisoning a few poor bastards. As for Kefka, he's just f-ing crazy. Kuja was obviously inspired by him.

I've read some reviews of Dragon Quest VIII, and the villain, Dhoulmagus, is made out to be "on par with Sephiroth" in terms of sheer evilness. Is it just me, or is he shamelessly copying Kefka?