Welcome to 1789...

Gave this even a bit more thought. Unfortunately in your scenario, Saint-Just, who has been hunting and soldiering since his youth, who is very, very comfortable with guns, who has led armies in battle riding in the front lines, misses his best friend's head at point-blank range. For some unknown reason, he doesn't aim for the heart or the temple (as would anyone in their right mind if they were trying to kill someone), but instead fires into Robespierre's jaw by mistake. I guess if we are looking for the absolute worst marksman in the room, we'd probably have to default back to Maxime trying to committ suicide and failing. Or, there is always the theory of the just-barely-trained guardsmen seeing someone lift a gun, and just randomly firing in that direction - and hitting Robespierre from a few feet away by mistake. I give up, you decide... :)