I'm just back in from walking her, a border terrier cross, and I'm really shaken and upset. She was off the lead, not long arrived, quite excited, he was walking towards us minding his own business, and she just went for him- jumped up and bit his upper arm, snarling and growling. I got her off him immediately by hitting her with the lead and shouting "Get down!", re-leashed her and apologised profusely as you do, checking he was ok. He was- he had a thick coat on, and it wasn't ripped, but you could see where the fabric had been gashed at. He was uninsured, but obvious and rightly extremely pissed off.
She is a bit under the weather with a skin infection she keeps needing retreatment for- the vet says its like doggy MRSA basically, and very resistant to antibiotics, so it gets better with them for a while, but keeps coming back. She bites and scratches at her skin a lot (good diet,vet says I'm doing nothing wrong)- perhaps that has made her irritable?
She has done this once (well twice, i found out, as DH says she went for a teenage boy's leg once, but dodnt mention it to me at the time 
) before, again to a lone man walking towards me. It was more like jumping up aggressively and growling rather than biting though.
I paid for sessions with a specialist dog behaviourist, who helped me work on her recall and feeing that I was the protector, not her. This was about six months ago. We practice the recall all the time, and I can mostly get her back on the lead if I see another dog approaching, because it is more usually strange dogs she is arsey with, rather than people.
I don't really know what else I can do- she is extremely active and tireless, and I can't see her having much pleasure in life if I can't ever let her off the lead 
. My mum was with me when this happened, and she thinks that the only responsible thing to do is to have her put to sleep. I can see her point- but is this an over-reaction? I have worked really hard these last months with her, but if she is going to be this unpredictable, I can't even have her on a long line in public, can I?
Please tell me what you think I should do- and please don't have a massive go at me either if you think I've done something wrong, I'm really upset and a bit teary writing this: I just want to know what to do for the best.