True, they are a powerful, intelligent dog. that require a powerful, intelligent trainer/owner. but to say that they were bred to kill is wrong you can not breed to kill you can breed to to be effeicent at what you do they are strong, intelligent loyal dogs, dominant dogs, and have been selectivly bred to be that way. so as an owner you have to be the alpha to a very dominant dog and not many people have the time and patience to be that.
now as for Mastifs it is not a breed it is a class that includes Newf's, st.bernards, bernese mountaindogs, bull dogs, danes, caine corso, and on and on. and depaending on that breed you get the bred characteristics.
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on the same note no dog should be at an off leash unless the owner has control in any situation. you call the dog should come. if not keep the leash on you don't have control.