What caused so many people to die?
Tana d'Vei Eliyahu Rabah 12: Because the Sanhedrin did not go around Eretz Yisrael to teach Derech Eretz to the nation, 70,000 in Giv'as Bnei 1 Binyamin died. Rather, the Sanhedrin turned to their vineyards, wine and fields, to avoid the toil.
Malbim: Because Binyamin killed 40,000 Yisraelim in the first two days, Yisrael took vengeance to destroy them. If not, they would have killed only the sinners.
It is reasonable that in addition to the 25,000 soldiers, there were another 45,000 in Binyamin (mostly women and children) who were killed. However, if the Sanhedrin is to blame, we should count also deaths in Yisrael! Yalkut Shimoni (Shoftim 68) says 'in Giv'as Binyamin.' We find that 40,000 soldiers died from Yisrael and 25,000 from Binyamin (20:21, 24, 46). Perhaps the Navi did not record that another 5,000 Yisraelim died after the first two battles. Perhaps they killed 5,000 in Yavesh Gil'ad (there were 400 virgins there). However, why should we count them, and not the rest of Binyamin that died? (PF)