Two things will happen...

First, you will be cussed out with a lot of F words and told not to 'tell me how to keep my dog, he's a member of this family!', then you'll be told to 'get off the property right now'

Secondly, anything that happens to the dog will be blamed on you, now that the neighbor knows you have a problem with their dog. Dog gets sick? YOU poisoned it. Dog gets hit by a random car on the road? YOU ran it over....and now you've got a neighbor out to kill you.