Lavos is just a specimen from an animal species, which parasitates a planet then reproduces then destroys its planet and finally sends its offspring to other planets. If Lavos came from Crono's planet, that world would already have been infected and ruined by its parent...
The section you are refering to is:
Robo: It possesses the DNA of every creature that has ever lived! It evolves by obtaining record of the genes of all living things of the planet.
Lucca: I understand. This jerk's objective...
Over a long period of time, (Lavos) gradually parasitizes a planet, collecting portions of the planet's constitution, which it relies on.
Possessing those genes, the descendants born on the Mountain of Death go to another planet, to repeat the cycle.
It sucks this planet's life over a long period of time, evolving while remaining dormant.
While this is an accurate description of Lavos' nature at the time of battling with it, it says nothing about why Lavos is like this. The explaination that lavos is a destructive animal or weapon is to simple an explaination for the power Lavos has over time/space and the unsual benefits to humans that Lavos exsistance brings. Remember Lavos well could have been corrupted by the DNA of humans and other beings into assuming its destructive form later in its life.
Again what I'm saying is that there is no established purpose for Lavos before his arrival, and I believe that this indicates the possibility of a more complex reason to why lavos is here.