There is absolutely no justification in killing this dog. He has nipped - yes, nipped, not bitten, read Dooin's intelligent remark about the difference. He is a young dog, little more than a pup, His strong cross is with a herding breed which are renowned for nipping. The OP is a novice who, respectfully, should never have been allowed to adopt him given her personal and family circumstances. The pound and the dog warden who homechecked her are at fault here.
Remember too that I have spoken to the OP and I have it from her, as she was with her lad at the time and saw the incident, that in her own opinion this was not a bite situation but more of an attempt to herd/seek attention - which, as I said, is very much a characteristic of the breed when young and untrained.
Apart from that, this is what I do, my life's work if you like. I have experience in rehabilitating dogs and work alongside those who do so day in, day out. with magical talent. If it wasn't for the fact that I have 3 dogs of my own I would take in this boy with my children and my cats and other dogs, without hesitation. I'm confident, from what I have learned, that he can be rehabilitated without a fraction of the difficulty which some dogs have presented to rescue and I and who have been turned around nonetheless.
I think that this experience and that of those who I deal with in rescue is just a teeny tiny bit more reliable than that of some of the armchair experts on this thread.
And btw, to the "kill him" brigade, I told you to fuck off. I don't think I've called you names, as has been alleged.
Yet.