What's the point of a one-man stoning?
Stoning takes advantage of crowd psychology, so that a barbaric society can execute it's chosen hate target whilst minimising guilt and remorse and hence any chance of rejection of its principles by the executors. Any one member of the baying mob hurling rocks can excuse themselves by the fact that their contribution to the overall suffering and death of the vicitm was very small.
Even as recently as the world wars, firing squads would be given randomly allocated blank and live cartridges so that any one member of the execution party could delude themselves that, statistically, they weren't likely to have been "responsible" for the death of a fellow soldier by obeying their orders; they might have been the one to fire the blank.
To anyone sufficiently intelligent enough to have a basic grasp of social dynamics, the fallacy and the danger of such thinking is as obvious as is its effectiveness at manipulating those who aren't.
Evidently the fundie in question is nowhere near intelligent enough to grasp even vaguely the point of stoning, and simply did it dogmatically because it's the traditional, prescribed method. When only one person carries out an "execution" by stoning, it's just vastly inefficient (and hence all the more needlessly brutal as the victim dies slowly and in agony from multiple blows) and doesn't even have its original advantage of "diluting responsibility". Mobs of self-righteous hotheads are collectively dangerous, vicious and stupid. This person is even more vicious, even more stupid and hence even more dangerous.