Its possible that the people of Guardia were convinced that their kingdom was outdated. With the ongoing modernization, perhaps Porre's leader managed to turn the tables on Crono and Marle. And would Crono and Marle want to killl their own people? I doubt it very highly. I imagine that they would defend as best as they can and escape given the chance.
I really like this theory. I thought about it too once but the problem with it is that we clearly see Truce in flame with villagers dead and scattered around... If the people were on Porre's side, who would have massacred them?
Maybe it's the guy who stole the Masamune who killed them, he would have been driven berserk and not be representative of Porre as a whole. He would basically be the "incident" that Masato Kato mentioned in the interview.
Or maybe Guardia is actually responsible... If Porre gained a new, charismatic and aggressive leader, maybe Guardia did too? This wouldn't have been a new King of course, but maybe a new general or a new chancellor... Then, seeing the population going revolutionary, this general could have ordered the army, or a small rogue faction of it, to massacre the rebels purely and simply (stuff like that happened in real history). The King, Marle and Crono would have been in the middle of all that crap and not know what to do... There would be an awful lot of different factions in Guardia, which would explain why Radius in 1,020 AD says that Guardia is not the "peaceful country" that it used to be.