Beast or Man Best Friend

Recently my mother in law was bitten by her beloved dog ROCKY..... (a Rottweiler)
After the incident, there were many questions popped out...
Will this happened again? Why the dog react this way? Is it because to the dog was not trained?
Or is this type is too agreesive ? Should we keep other type like Golden Retriver?
Should we get a trainer to train him?
Could he be trained at this age?

Until all these questioned were answered....
I definely will stay away from "ROCKY" or else.......