Guys! This village was already shown to be filled with scumbags back in chapter 18, just read the first 10 pages, and it's showed that he knew from quite a while that the village was full of scumbags, he also has personal revenge with the villagers since they were also the ones who willingly let the soldiers, who were his friends, to die as monsters
Also for some who are asking if the hero let the girls die, look at the first page, there a hooded figure holding a lantern walking were it was showed that it was the place the villagers buried the girls as the page transition to white to black at the bottom, so it could be said that he either knew that the villagers buried their victims there, or that he used magic to find it, then he used his magic to see how the girls' corpses died and then used his magic to disguise as the older sister when he showed for his revenge