Why not do a google spreadsheet or something? Then you can generate graphs, insights, &etc. much easier when the year is done, or even in throughout the year.
I would also recommend maybe getting in contact with NEU which has a substantially similar project. Although you have recorded more events this year*, they started earlier and have better presentation.
* You include events with four deaths, whereas they require five (four victims).
You have caught onto a key point. In fact you have identified why I got started with this. In the Old Days, the FBI would consider four dead a mass killing. Then they began to not count the traditional suicide. Then they began using the term active shooter to no longer count the most common form of mass slaughter, a family killed at home by a parent.
Very simply they were moving the goalposts and changing the definition to minimize the count. That seemed wrong to me.
In much the same way, some people only count mass killing by gun. It seems to me people who are killed at home by a crazed father count. So do suicides to my way of thinking. So do people killed by arsonists and drunk drivers.
One of the things I have learned is that the rules of who you count matter quite a bit.
WASHINGTON (AP) — The number of U.S. mass killings linked to extremism over the past decade was at least three times higher than the total from any other 10-year period since the 1970s, according to a report by the Anti-Defamation League.