Digging Into the Data on Right Wing Extremism

After having a conversation with a friend I decide to dig into the numbers on police officer killings since 1965 as compiled by the Anti-Defamation League. In the process I discover that there's a lot of fairly obvious subjectivity to who those numbers can be interpreted, and the general impression that right-wing extremism is more dangerous is muddier than people think. 

It's a long one, but it's got lots of numbers so that makes up for it. Right?