I have asked this before. I is no real answer other than OP wants to.
He has stated before he has no political motive, it's just a hobby of his to compile a list.
(and since the list includes many deaths not involving guns, it's believable that his motive isn't a commentary on gun accessibility in the US, for example).
I had a theory he lost someone to a mass killing and is why he has such an interest, but then I have a fascination* in missing person cases but do not personally know anyone that has been reported missing, so my theory could be way wrong.
*Not enough interest to compile a list, but love to read about them and wish I could solve them/find them, etc and might be what I spend time doing when I retire or win the lottery.