Hi Mirannan,
I follow you, and it would be very dependent on what the officer initially saw.
The first thing I will say is that, in layman terms, an arrest is not us saying that a person is guilty of an offense... merely that the we feel the need to ask you some pretty pointed questions if you follow.
The second thing is contrary to what the media will have you believe, the law clearly allows one to act in self defense (of self or another). Common law, Section 3 of the Criminal law act 1967 and ECHR have it enshrined in them. Police have some further laws and powers but bottom line the above covers 'civvys'. The test is, was that self defense PLAN (That is Proportionate Legal Accountable and Necessary)
Okay to answer more directly.
If I was at one end of the street and saw person A at the other innocently walking down the street where he is set upon by a gang and person A handed them their backsides because he's a bit tasty, then I would still treat person A as the victim of the assault. (As long as his response was proportionate and neccesary, no kicking someone when they're down!). If that gang had weapons, then I would still treat person A as the victim even if he had injured the gang pretty badly, as because they have weapons their intent is clearly to cause major injury. I would also, as an aside, probably enter him for some kind of citizen award or recognition.
Now if I only saw half of it, e.g. the bit where all of them are scrapping I would be more inclined to lock them all up for affray (having a fight) and sort out who (if any) are the victims and who are the offenders in a controlled environment (e.g. a nice warm custody block). Simply speaking I need more evidence to figure out who the goodies and baddies are. If there are witnesses (who I might not have the chance to speak to there and then because I'm dealing with a scrap, but would speak to them later). I will consider their evidence and then release person A (Again as long as we're still PLAN) with no further action.
Now if only I saw the bit where Person A is standing over the gangs broken bloody bodies, and with noone to tell me otherwise, (and lets say the mere presence of the weapons were not enough to suggest one thing or another) I would probably treat the gang as victims and person A as the offender. HOWEVER it would certainly be on my mind to consider the wider circumstances and be open to de-arresting person A and arresting the gang. Just not necessarily there and then.
Consider the Tony Martin case (the farmer who shot the burglars). The law wouldn't have had too many problems if he had shot a burglar who was coming TOWARDS him, but the fact was he shot the chap in the back as he was running away and at that point no harm to anyone (You may have views on the chap getting his comeuppance, but that's an ethical debate, not a matter of law). Bottom line Martins actions were not PROPORTIONATE to the circumstances at that precise moment.
In relation to handcuffing, well depends on the injuries. We have a duty of care to people and I'm not going to handcuff someone who is badly injured if my first priority is first aid.
I hope that covers your question dude