Every person present was legally allowed to place that officer (and his boss on the phone, and every officer present) under arrest.
They are armed and are committing a felony in the presence of other citizens. They had every reasonable expectation that the officers will resist arrest violently and with deadly force.
The people there had every right to place him under arrest, and had every justification to fear for their lives in doing so. Force, up to and including lethal, was justified to stop this illegal kidnapping.
A fire extinguisher to the back of the head seems appropriate. You would agree it was appropriate if you saw an armed man robbing a gas station, so why would you deem it inappropriate to stop an armed kidnapping?
----citation 1----
Utah Code Title 77 Utah Code of Criminal Procedure Chapter 7 Arrest, by Whom, and How Made Section 3 By private persons.
77-7-3. By private persons. A private person may arrest another: (1) For a public offense committed or attempted in his presence; or (2) When a felony has been committed and he has reasonable cause to believe the person arrested has committed it.
----citation 2----
18 U.S. Code § 241 - Conspiracy against rights
If two or more persons conspire to injure, oppress, threaten, or intimidate any person in any State, Territory, Commonwealth, Possession, or District in the free exercise or enjoyment of any right or privilege secured to him by the Constitution or laws of the United States, or because of his having so exercised the same; or
If two or more persons go in disguise on the highway, or on the premises of another, with intent to prevent or hinder his free exercise or enjoyment of any right or privilege so secured—
They shall be fined under this title or imprisoned not more than ten years, or both; and if death results from the acts committed in violation of this section or if such acts include kidnapping or an attempt to kidnap, aggravated sexual abuse or an attempt to commit aggravated sexual abuse, or an attempt to kill, they shall be fined under this title or imprisoned for any term of years or for life, or both, or may be sentenced to death.