I hate to compare the two, but if a Wizard killed said Goblin with magic would you award an instance of Lawbreaker? Whilst not a great or entirely accurate comparison, both have a fair amount to deal with a characters inner-self, & both can bite you in the ass even if you didn't intend to kill with your power.
Personally, for a Goblin I would say it doesn't count. They are monsters, beings of nature. They lack the human capacity of free-will, which makes human desires oh-so-more, well, more. My own (admittedly completely unfounded) opinion is that it should apply to killing a mortal or semi-mortal, not a monster. But then, I have no real evidence to back my opinion, and I can easily see merit to the opposite opinion.
Nice post Richard, some interesting things you've brought up.
At the end of the day MacShidhe, go with what works best for your group.