Okay, so some say he isnt evil cause he is just eating, but than WHY DID HE RANDOMLY USE DESTRUCTION RAINS FROM THE HEAVENS!?
We don't know a lot of things about Lavos, mainly due to the fact that nothing much was really explained about him.
Of the few things that we do know, is that he fell on the planet on the year 65,000,000 B.C. and that he has been feeding on the planet's energy since then; that his power can be extracted and manipulated to a certain degree through the Mammon Machine, and that he makes the world a fairly inhabitable place for humans to continue living.
that kinda seems a bit even, i mean, he was done eating, so he coulda just left, and dont tell me DRFH is how he eats, cause his spikes dont have mouths on them.
We don't know if he is evil or not. If he is sentient or if he merely acts out of pure instincts. If he was created by someone as a biological weapon or if he is simply a really advanced creature in the process of evolution. We don't know what his real purpose was for destroying normal living conditions on the planet on the year 1999.
So yeah, like I said, we don't know a lot of things.
anyway, just why DID he do that?
This act was probably related in someway to benefit his offspring, the Lavos Spawns.