Hmmm.
I personally go with the whole 'residue of negative energy' theory; after all, it's made of Dreamstone, which must be exceptionally sensitive to the will and intentions of its user. It would stand to reason that after enough bloodshed and chaos the sword would be heavily tainted and in need of a spiritual cleansing of sorts.
This has probably been avoided by the nature of the users to this point, as regulated by Masa and Mune, as it's been pointed out. Frog is really an exceptional being in terms of nobility, kindness, and pure intentions - I mean, in canon he spares the life of his worst enemy, who he's spent most of his existence fighting in some form or fashion. (Also, personally, I think he must've faced a lot of discrimination as a monster-type creature fighting on the human side. My personal version of Frog actually has a fair amount of compassion for the Mystics.)
I really don't know if the fact it's enhanced by Lavos' energy in and of itself is such a bad thing. Power is just power, really, and CT on its own has several examples of Lavos' energy being used against it (Masamune, pendant, etc.). I suppose an argument against this would be Zeal, though, but their tapping of Lavos' energy was obviously on a much higher scale.
