I think leaders usually will commit violence without a follower, I just think it will look different. I think the leader in Columbine would have still been violent, I just don't think it would have looked the same. I think leaders often need someone to be a part of their game. They need others in their plot. The leader would probably be violent in some sense, but I wonder if it would be more reactive...rather than this twisted plot.
Incidentally, I've noticed in a lot of these pair murders...leaders also need someone to blame, or at least to minimize themselves.
A lot of killer duos, both child and adult, exhibit signs of a violent nature prior to meeting up with their partner in crime, but most only escalate to murder once that partnership is entrenched. Of course, there's exceptions (as there always are) but I'm talking a majority of cases I've looked at.