We assume that in the brief ending that it was one of the forces of guardia that wielded it agaisnt porre;and that porranian took it. and thus it turns to evil because the holder has a evil heart.
Any alternate therioes on this?
I must take issue with this, unfortunately, though it is more a matter of specifics than anything else. The person holding the sword may not have had an evil heart, and may not have been an evil person at all (evil in the human sense - as I see it, all people are inherently evil as it is, thus no good people exist... but that is beside the point and irrelivant here.) You see, Masa and Mune themselves profess that it is not who wields the sword, but what they do with it. Thus a person who is by nature good, who even fights for good and thinks they are perhaps avenging or zealously defending Guardia, could have darkened the blade, simply by ruthlessly slaughtering those he saw as evil and as his enemies. The bearer of that holy sword must be very careful, then, in some ways like the Jedi in Star Wars must be. Whoever wields the sword affects it with his actions. Thus, even if he acts with good intentions, if what he does is evil, the sword is darkened. As we have seen by the example of Radius, this can in turn affect the wielder to further deeds, these being purposefully evil, further darkening the sword. A downward spiral that, in some respect, is similar to the spiraling path down the dark side. Thus, using the twin's own words, one may assume that whoever wielded it was not by nature evil, so needn't have been Magus or Lynx or whoever else has been speculated by nature of their dark natures. It may simply have been the knight champion of Guardia, such as Cyrus was on a time.
But as for who carried it in that last battle... personally, I have neither clue nor theory. Most time I see things one way, but know well enough what the likely intent was by those who made the game. It is not so plain here, however. It may perchance have been that hero Crono, but may just as well have been some other warrior of the land. And whose hands did it fall into? Some have said Lynx, others Magus. But it may just as well have been some other soldier and, perhaps, that soldier was not evil at the beginning. Rather, once dark, the sword acted akin to the One Ring of legend, possessing its owner to work its dark and murderous will, and passing from master to master in this fashion. It certainly fits with its hold upon the mind and spirit of Dario, does it not? And see, then: it may have been any of Porre that could have done this, even a common warrior carrying the sword abroad as war-spoil, like Isildur taking the ring for his own as wergild. That is what I think of official accounts. And for once, I have no personal theory on the matter either, and never speak of one in my writing: I step about the issue, and when Schala asks Crono of the matter, he says that he knows nothing of it, having left it behind in the ruin. Thus, my opinion on the matter is undecided.