I had a 12 year old Labrador put down in 2016 - funnily enough he was called Nero as well, because he was black - at the time he too had become grumpy, he was also becoming incontinent, but the last straw was when unprovoked he took a lump out of my son's leg.
A child is ALWAYS more important than a dog; he may have had a year or two left in him, but I wasn't prepared to take the chance of it happening again.
People like margie need to think a little, if that's possible, before branding these people scum; the owners were doing what they thought was right, the growls may have turned into something more, so they did what they did because they were putting their child first.
I don't blame them.