According to the names of his attacks, Lavos would really be evil : Evil Emanation, Shadow Doom Blaze, Shadow Slay, Span Death, Crying Heavens, Dreamless, Evil Star. Only "Invading Light" doesn't sound evilish.
Anyway, the existence of the Frozen Flame shows that Lavos must have some conscience : the Frozen Flame is conscious, and it serves as a mediator between an user and Lavos (it would be hard to mediate anything if Lavos was a mere animal).
As part of the Devourer of Time, Lavos shows sadness, sorrow, despair and hate, so it prooves that he's conscious and has feelings (although negative ones).
He is master of magic so he probably knows every spell and attack which can cause magical damage. From the game we can see that magus is strongest in magic, maybe it`s because he is using shadow magic which maybe is strongest of all magics and because this magic is believed to be evil it`s spell names are evilish. So if shadow is trongest wouldn`t it be logical for Lavos to use strongest magic?
Lavos isn't the master of any magic. He's just a leecher off the planet. My theory is that while he's sucking away at the Earths energy, he is also sucking the elements. In this case, he would have transfered the elements to the humans around 10,000bc if they came in contact with the frozen flame. These elements were stronger than the CC elements and were acctually magic. Hence, everything Lavos gave to the Zealians were the planets gifts without slow evolution.
The elements of CC are from Dinopolis which was pulled back like Chronopolis was.
Back to the Masamune.
I believe that the Masamune was turned evil... By Frog. Why? Look:
1)Frog was giving off hate emotions towards Magus, this could have been absorbed by the sword
2) The sword when attacking Magus could have been absorbing Magus' hate for Lavos and Frog, not to mention his own dark magic and feelings.
3) You'd think that a big evil hateful ghost that empowered the Masamune would leave behind a trail of evil feelings into the sword, especially if the hate feelings have build up over 400 years.
This is how I believe the sword came to become evil. The only counterbalance I see are these:
1) The sword only remained good until Frog died and the quest was over, and Frog made up with Magus at the North Cliff.
2) Frog kills Magus, Sword absorbs heaps of evil energy but since Masa and Mune are awake they are controlling the sword with Frogs good intentions?
It's even mentioned in CC that Masa and Mune were asleep while they shouldn't have been, and that Doreen was pis*ed because they forgot her when they fell asleep with the blade, so thats why they all joined together to make the Mastermune.
I'm sleepy now since its 12.26am and my brain hurts x.x