Who was the cruelest teacher?

Impossible to answer - each person can only go with their own candidate. Buy this is mine:

In his wonderful historical non-fiction novel "G.I.", historian Lee Kennet (R.I.P. March 13, 2011)

wrote of an American G.I., a Sargent, in liberated France during World War II. Fortified with the near-lethal Calvados brandy of the region, he attempted one night to teach his elderly French host to speak English. After a short time, the Sargent drew his .45 sidearm, proclaimed the Frenchman "too stupid to live", and shot him. The old Frenchman lived, but sued the United States Army for damages, which were quickly paid. The Sargent drew thirty years in Leavenworth (but, typical for the Army, served about 6 years).