I am sure I am going to get flamed for this... but... short of the fact an 18 year old died, I do not see a tragedy here at all. I see a tragedy avoided. I see that if nothing else, the tools of self defense were used efficiently and successfully, and I would like to assume that some mindset and hardened security led to the successful protection of the mans loved ones.
Google Steven Spader (17 year old, boy scout, "good kid", "peaceful", according to family and friends) He was charged with 2 counts of murder, and 1 count of attempted murder... the only reason it was only two counts of murder and not three is because the 11 year old girl he hacked away at with a machete had the presence of mind to play dead.
So if you consider the family who was featured in this post as "Family A", and the family I referenced as "Family B" there is only one tragedy. The teens in Mt Vernon NH "Knew before they entered the home, that their goal was to kill the occupants...." They wanted to see what it was like to murder someone.
Given that, any intruder who is attempting to break into your home, of any age should be considered a grave threat and treated as such.