It is definitely open to interpretation. I have the feeling that he's supposed to be evil, but I doubt the developers thought this far. Or perhaps they did? Maybe they wanted it to be open-ended. A lot of the things in the Chrono series are. (Though some may be due to the developers overlooking certain things, it's still open-ended...) I like the idea that Lavos is a "varelse", that he's on a completely different plane of thought. We are like squares trying to perceive the depth of a cube. His thought process is so different than ours, so that's why we never hear him talk.
Curiously, we never hear the planet (the Entity; I like calling her Gaia) "speak" either. I have the feeling that Lavos and the planet are on the same plane of thought. (a.k.a. tons of times higher than ours. :p) This leads me to support the idea that Lavos is another "Entity" that has "gone bad", as someone suggested. It now only wishes to be a parasite that consumes the DNA of all planets, and lay them to ruins. It would explain why it's so powerful, and how the planet cannot fight it. She probably tried, but it was stronger.
Or something. I have no clue. Since Lavos is varelse, and we cannot communicate with it, we cannot see its higher motives. It's an interesting theory that Overlord proposed, but Lavos is definitely sentient, and I do feel that it is consciously making a choice to hurt the planet, a being I feel is probably on the same plane of sentience as Lavos. But, like I said, we can't communicate with him, so it's all open to interpretation.
My standing is that Lavos is evil, simple as that. But that's not to say there are other plausible theories floating around there.
P.S. Yeah, Occam's Razor can't really help us on this one, so that sucks. :( A word from the developers would be helpful, but I think that it's quite possible that it's meant to be open-ended, so they couldn't help in that case. But...