I am very sorry for the loss of an obviously beloved pet but as a person who has experienced a vicious dog attack that required lengthy recovery, physical therapy and permanent injury, I tend to lean toward empathy for the father who had to make a very snap decision.   Being attacked by a dog is a terrifying experience and sadly too many dog owners are irresponsible and do not  keep their pets properly contained nor invest in the time to train.  How was this new neighbor to know there was an invisible fence?   I empathize also with the dog owners grief but putting up a 'dog killer' sign in their yard sadly does not reflect very well upon them.