Agreed. Of all the ways to try to figure out motive, using voter registration alone is probably the absolute worst.
I registered to vote as a freshman in college. I registered as a D, primarily because an R was in office and I wanted to choose who I thought would be the best D if the R was to be beaten in the general. After school, I worked for years for an elected official who was a D. I maintained that registration so I could vote for my boss in the primaries. The county in which I live is primarily (80% or more) R. When the old boss was finally defeated many years later, I changed to an R so I could have a legit voice in my local elections.
Looking at my registrations, would anyone know anything real about my electoral preferences or my political preferences? No, not really.
I admit it, my registrations have always been about maximizing my (minimal) impact in any given election. It says very little about my political leanings, and far less about who I would shoot, and why I would shoot them, if I were disposed to take another person's life, (which I am NOT, just to be clear.) Heck, it doesn't even tell you who I voted for in any given election! It is a poor independent factor to make any consequential determination about a person.