Not plausible at all.
1. The bandits seemed pretty pissed, and if they saw the St. Johns, they probably would have killed them too, which makes it an unnecessary risk.
2. The St. Johns wanted to eat Mark, and the meat goes bad if they die first. Shooting at him with an arrow is a big risk, since they might have killed him before being able to carve him up for meat. They didn't need him to be injured in order to take him aside and start carving him up. It made a convenient excuse, but it wasn't necessary, which would make shooting him an unnecessary risk.
3. The St. Johns turned on the electric fence, which meant that if one of them was outside of it, they would have to run back to one of the entrances with: an unknown number of zombies outside (the whole reason they have the fence working in the first place); an unknown number of bandits outside; and while running the risk of being seen by Mark and Lee. Again, more unnecessary risk.
There is no reason why they would have done such a thing. It makes no sense and it far, far too risky.