In Florida, state law (FSS 901.18) allows LEOs/LEAs to direct any other person, whether or not also an officer, to assist in execution of civil and criminal duties, including making arrests. Florida Statute 843.06 directs any person so required "in the name of the state" to provided such assistance. Failure to do so is indeed a misdemeanor offense. Compliance places the assisting citizen in the same position as another LEO, in fact, as a deputized citizen under the law, for the situation at hand.
Now, that being said, there is no clear language stating exactly what assistance can be ordered, or how it should be ordered. Does the simple demand "Help me!" count and meet the requirements? Or does the officer need to make it more clear that he/she is indeed deputizing the other person and that the person being deputized is now obligated to assist? Does the law permit the officer to actually direct the assisting person's actions?
In the legal side of this, it would simply be interesting to know if the law considers the commands for the citizen to "shoot him!" to be enough to count as having directed him "in the name of the state" to provide assistance. The investigation into the shooting itself would determine if it was justified, but I doubt the command to actually shoot could be seen as an order the citizen would be required to follow. After all, a second police officer arriving on the scene would not face charges if he, instead of shooting, simply tackled the suspect.
In this case, it appears the citizen acted in the best way he could, given the incident as I understand it to have taken place.