What caused division between Sunnis and Shies?

After the death of Hazrat Muhammad(SAW) Muslims began to argue about who should be the next leader of Muslims, some of those who favored hazrat ali were called shias and those who favored hazrat abu bakar and hazrat umer were called sunnis vut all three of them became khalifa one by one in the end.

The unfortunate incident of the merciless martyrdom of the grandson of the holy Prophet (MPBUH) along with the members of his family at Karbala caused the split in the Muslims - Shiites and Sunnies, though the incident was equally condemnable for all the Muslim Ummah. It is equally regarded shameful and hateful by the Sunnies. No Muslim justifies it. They have the same regard and love for the Ahli-Bait as anybody else may claim.