Would it be wrong to 'fix' someone else's dog?

I am dog sitting for my neighbors memorial day weekend. They left me with their inbred male pug. My neighbors are country folk and for whatever reason lack any sort of compion towards animals - I have seen this demonstrated on more than one occasion. They have 3 female dogs (jack russel, 2 lab mixes) that all live outside. They feed these dogs but rarely interact with them and they all have fleas and worms. The pug is the only dog allowed in their house. THe male is constantly (2X per year) impregnating these feamles, so there is a constant supply of unwanted puppies. I have tried to find them homes before and even adopted one once (which I have since found has genetic defects thanks to the inbred father) but typically the puppies end up dying or mysteriously disappearing. I feel like I have my chance this weekend - I want to secretly get their dog a vasectomy. However, I do feel a little twinge of guilt. Would it be completely morally wrong to do this behind their backs? Or do you think the harm they are inflicitng on these animals warrants the action??