An English Mastiff that has attacked and killed small ponies, Will it be capable of killing people?

A breeder has given a 3 year old Mastiff up for adoption rather than putting it to sleep after it has killed and disemboweled 2 small ponies. My sister in law has taken this dog; who currently has 4 Mastiffs as well as 2 small children and 2 elderly women living under one roof. A vet told her that there is no danger, but I have hesitation in accepting his opinion and believe an animal that has tasted blood and killed will repeat the action again. Any Veterinarians out there that can answer this question will be greatly appreciated.