I can't understand why this thread has got so aggresive and personal.
I am deeply saddened by the pensioner being killed by his own dog, how positively awful. I havent been able to find anything that comes up with a reason for the dog flipping out, was it an otherwise good tempered animal? It may well have had some sort of neurological problem, but who knows.
The OP raised the Q about dogs off leads in public places, and freely admitted that it is the image of the rottie that scared her. I have to say, to be fair, that actually, rotties ARE quite scary, irrelevant of the hysteria of late. They just have such a penetrating "stare" and trust me, if a rottie is staring at you, its not a good thing. My rescue rotweiller used to do this, he would fix his eyes on someone if they stared at him, it was was quite unnerving.
Really it is a matter of common sense, if you have a big boisterous dog, keep it on a lead. If you have a quiet obedient dog, regardless of size, let it off the lead where appropriate IE: quiet beach, woods, fields. I dont think it is appropriate for dogs to be off lead in public parks where young children are playing, regardless of size or breed.