Baraka
While Mortal Kombat characters have always been pretty brutal to each other, Baraka has always seemed to be the most cruel character. Seriously, that guy lights innocent civillians on fire and then throws them at you, laughing as they scream in agony and run into you, causing you damage. His Fatalities have actually never been that brutal until recently, but then again, the developers specefied that they had a few ideas that were
too brutal, even for Mortal Kombat, such as a suggested fatality in which he would slice his opponent's midsection open and grin with his sickening maw as their innards spilled out onto the ground. He's a heartless mutant bastard, and I love him for it.