Masamune (Corruption of)
How did the Masamune become corrupted after Chrono Trigger?
Masa and Mune state that until 1020 A.D., they were asleep within the sword. In addition to this, they state at the Denadoro Mts. that only the strong of will can wield the blade. Perhaps, due to their sleeping, people with inferior moral fortitude and strength (contrast to Cyrus or Frog) used the sword, the result being its corruption. Frog had the necessary mettle to use the sword to slay Magus with soberness and purpose, while Radius and the others perhaps were overcome with ambition and could not restrain their desires to use the sword for selfish purposes. This does not take into account the apparent power of the evil Masamune to outwardly corrupt the wielder, however.
This draws on the Reflexive Power theory outlined at Masamune (Reaction to Magus). Masa and Mune state that until 1020 A.D., they were asleep within the sword; the battle with Dario eventually woke them up. Now, considering that the Masamune is capable of expressing the will of its bearer and also absorbing Lavos energy (and perhaps other types by extension), perhaps the Masamune was stolen during the Fall of Guardia and used to commit horrible deeds. This imbued it with a negative energy that remained and also influenced the wielders of the blade from that point on. After all, the Masamune is seen in its original form stained with blood in 1005 A.D. in the PSX cutscene, and is grabbed by the man who slayed the last protector of Guardia. Additionally, Garai seems to possess the sword after he's slain by it, supporting the notion that it can absorb negative sentiments. Another idea is that Frog fell to a negative fate after 600 A.D., and that the negative sentiments were etched into the sword in this manner.
In Japan, there is an old myth that basically states that swords had souls, and if you didn't clean your blade after killing someone, the sword drank the blood and became demonic. (This myth is tapped frequently in Japanese media, like Soul Edge from the Soul Calibur series and Tessaiga from Inuyasha.) This could apply to the Masamune, since it's stained with blood. Masa and Mune could've gone to sleep, since their destinies were completed, and with the blood of the fallen, plus the tainted, if not evil, ambitions of the blade's weilder boosted by dreamstone causing it to go demonic.
Masa and Mune state that until 1020 A.D., they were asleep within the sword. Also, Dreamstone conducts the energy of Lavos. Perhaps while the Masamune existed, Masa and Mune consciously protected it from being negatively influenced by Lavos's energy (note that Lavos energy doesn't necessarily have any outwardly negative consequences). After Lavos was defeated, they went to sleep, feeling their job was done. However, they did not know about the Time Devourer, whose emanations were able to corrupt the blade. However, this theory requires that the Time Devourer still produced emanations comparable to Lavos's energy, and that these emanations could cross the boundary from the Darkness Beyond Time to the outside world.
Some have contested that since Melchior probably finished the sword in the Lavos Timeline, as opposed to its completion by being plunged into the Mammon Machine by Crono later on, that the original blade was stable and the new one was corrupted due to the nature of its christening (with Lavos energy). However, this is precluded by the time travel rules set out by the series. What if Frog is at the End of Time when the Ocean Palace Incident occurs? Does the Masamune "reflow" immediately and become the new version at his side? Chrono series mechanics debunk this possibility. The sword also seems to be in its original form at the Fall of Guardia cutscene, as it appears as it does in Chrono Trigger.
From: Theory (Objects)