Kefka's accomplishments are arbitrary compared to the reasons (or lack there of) he tried to accomplish them in the first place. Killing lots of people was just a vehicle for portraying how evil he was.

As previously noted, Kefka is the most evil villain because he kills for the pure joy of killing, rather then killing for a purpose. Any villain could have killed far more people and devastated far more worlds without being as evil if they had done so for a concrete reason.