All dogs can be vicious and can attack but pittbulls have been known to kill when they do. They have powerful jaws that can rip you apart where a Yorkie can only bite. Certain large breeds, for instance "pittbulls" and "rotts" are much more likely to kill that little dogs. Many families who have had their family members killed or viciously attacked and injured have said they had the dog since they were puppies and it never displayed an aggressive behavior, but low and behold the dog attacks and you have those that defend these animals probably just like the families that now have dead loved ones.