Sarah J - Good summary. I'll tell you about a story, if you or anyone else can be bothered answering it, about a situation that potentially may of potentially got very sad/disturbing and ugly along time ago(10years ago), but thankfully it didn't escalate. But I've always wondered what would have been the result, had it escalated legally.
At a bus stop in Sydney, on a nice day, nothing doing, a man and his young daughter aged 2-4, were standing at a bus stop minding there own business, when out of nowhere a drunk fairly old man (60-65) approached them. Was normally dressed, but I'd seen him around as I used to live in that area, he always looked normal, probably a homeless man but I suspected he was unemployed/or on a pension, or no fixed address(maybe living in a boarding house) and had problems with alcohol(could of been an alcoholic or had a drinking problem as I'd seen him drinking at bus stops/park benches etc, but always normally groomed and dressed smart casual eg jeans/clean shirt and washed not dirty etc). Anyway he walked past, and either he thought the little girl got in his way, or he for whatever reason lost it at her and starting verbally abusing her unprovoked, I will say highly agressively "lost the plot" to almost deranged if not possibly deranged levels. The dad was caught "completley off guard", the girl was frozen and crying, the dad was intimidated may if said leave her alone or just said okay mate, all this happened in front of shocked by-standers". I like the dad had immediate concerns for the girls safety, but were both caught off-guard, nervous didn't know how to respond. In a instant, I considered attacking the "old man" and hitting him from behind eg(a swinging arm), I was disturbed and emotional by what I saw,and had concerns for the child's safety.
But I refrained fearing I would either get hurt by attacking the man, as he was the same size as me and in an angry mood, or even if i hurt him and he hurts me, will we both be charged with assualt, or what if I hit him from behind will i get charged with "murder or manslaughter" if he dies and found guilty of either, knowing if that were to happen my defence would have to be I was concerned about the "personal safety of the child". All this happened so fast i had 30seconds to respond, as the incident went for about 1 minute max. The man then in an angry mood, then lied down on the grass as it was next to a park, and was rolling around angry and I think emotionally disturbed/shocked/upset by what he had done to this "child". Then the (dad+child), me and everyone else got on the bus, and that was that, the end.
But yep I have wondered what my defence would be if that had happened and I'd hypothetically hit that old-man, I suppose it would be along the lines of stories like if someone breaks into your house eg like in the US and the person shoots or stabs the thief, sometimes they go to jail(eg unecceasry force to contain the situation), or if someones dog at the park attacks you and you get angry at the owner and he gets angry back, and his dog keeps barking at you, intimidating you and protecting it's owner and you hit the owner as a result of being intimidated by both the agression verbally of the owner and the potential physical attack by there dog a strong dog (eg german shepard/pit-bull).
But yep my main defence would have been concerned about the "personal-safety of the child", if I'd have killed the old-man or caused serious injury to him eg broken jaw/nose/fractured skull/brain damage/put him in a wheel-chair eg paraplegic/quadraplegic etc.