Its a good origin for his insanity but its not really a motive if you think about it. He never uses it as a reason for why he does it what he does, though logically it may indeed be implied due to its nature.
I think it comes down to interpretation of "Best Motive" I feel it implies a good reason for doing what they did, and regretfully, though I feel Kefka is a wonderful villain, he only does what he does for personal amusement. He enjoys watching others suffer. Technically, Kefka has no real motive, he didn't even really strive to be a god like other villains did, it just fell into his lap. The other villains had goals, though many were pretty silly, I feel only the three I mentioned had compelling goals that made them unique villains and antagonists.
Despite that, I would say only Kuja of the three is actually a good villain. Vayne is arrogant but his goals are somewhat noble if a little self serving in their execution. Xande is just a coward. Kuja has a sympathetic motive but his personality and actions still make him a top class villain despite sympathetic intentions. This is where Kefka and Ex-Death come in. Their origins are strange and perhaps not explained clearly but lord knows they are top class villains in my book. Their origin explains why they are the way they are but they do not define themselves by it, like Sephiroth or Zemus did. Rather its in their symbolic nature that these two remain so enduring as villains. :)