If we're going to argue semantics...

Quote Originally Posted by United Nations CPPCG Article 2
Article 2 of this convention defines genocide as "any of the following acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group, as such: killing members of the group; causing serious bodily or mental harm to members of the group; deliberately inflicting on the group conditions of life, calculated to bring about its physical destruction in whole or in part; imposing measures intended to prevent births within the group; [and] forcibly transferring children of the group to another group".
So by international law, Sephy and Kefka committed genocide, cause they are directly responsible for the death of two races, and it was intentionally done by both. Course, most of the FF villains are genocidal maniacs, but this was just a comparison of the two and its not like neither of them don't share traits with other villains in the series as well.