The Netziv (introduction to Haamek Davar on Bereishis, and SHU”T Meishiv Davar #44) says the sinas chinam that destroyed the 2nd BHM was the phenomenon of tzadikim and talmidei chachamim suspecting their colleagues – whose opinions or practices were different from their own – of being “Tzedukim and heretics”, and hating them for it. This eventually led to actual bloodshed.
The Netziv then says “HKBH cannot tolerate such tzadikim”.