murdering this man was not just wrong, it was stupid.

Depends what your objective is. If your goal is to accelerate political violence and set Americans at odds to an even greater degree than they already are, it's completely rational. I have no idea who did it; it could be domestic extremists, foreign actors, cynical strategists. It might be some isolated murderous person with a chip on their shoulder who totally hated Kirk, but that seems like the least likely possibility because of the fact that they've made a clean getaway - 12 hours with no CCTV imagery or even a good description is unusual for such a public event.