2011 kia Sorento bad mechanic or normal problem?

Car died while driving, replaced battery car drive for 2 1/2 days then dies again. Replaced alternator saw tensioner bolt was broke bought replacement assembly for tensioner call mechanic to install he removed old one installed new one 6pm in the dark spent 15min cost me 160 bucks. I started car belt was torn but belt was less the 6months old and had very little miles on it. I should of started the car before mechanic left but I didn't and the belt was torn and he refused to compromise with me to install new belt said he don't work for free and he installed the tensioner pulley perfectly. But belt was perfectly intact no tears no ware before this mechanic installed my tensioner. The tensioner had to be moved and loosened by another mechanic belt is fine. Did this mechanic cause the belt to tear by not allowing the pulley correctly? Mechanic said no way and he won't work for free so wouldn't help.