although a lot of enemies seem mad like zeromus_X said, most of them make sense.

garland turning into chaos is the 4 fiends using their powers.

i dont know how the emporor took over hell but oh well.

zeromus. i have a theory on zeromus but after playing the game im beginning to think that hes just some random hate monster.

neo X-death is the void with X-deaths powers. X-death had partial control of the void but neo X-death is the void and has full control of X-deaths powers.

kefka is a giant magicite junkie.

sephiroth is the power of jenova and sephiroth combined.

ultimecias form are usually all about her power. she used her powers to create greiver and then to junction on and when she died her powers lived on to create the final form. ive explained my theory on what the final form is a lot of times so i wont type it again.

necron is believed to be the centre of the lifa tree but he could just simply be some sort of random god like creature who has always existed.