The fastest would be with kong. You can edit the alignment by typing a number from -100 to 100. whole numbers will be good, negative will be evil. DON'T EDIT IT IF IT HAS A LETTER IN IT! I tried that and it seriously messed up my game. make your creature eat something first and exit out of the game. Use that with caution.
The other option (which is pretty quick) would be naturally. If you teach him early or when he's neutral it's pretty fast. It's alot faster to teach a creature to be evil than good.