You are right in that stabbings feel much more personal psychologically, but I would imagine a murderer would pick the most sure weapon if it was entirely premeditated. It seems as though the guy had no intention of hiding the killing.
I’m sure many of these extremists were either charged with a felony before or involuntarily committed to a mental hospital. Both mean that they are federally banned from guns for life. I think many of these murders are out of strong rage rather than calculated killings, but they’d still probably prefer to use a gun if they had easy access.