Is a citizen justified in disarming a policeman who is applying a taser to a suspect who committed a misdemeanor, but who is at an elevated risk of dying due to health conditions of which the police may not be aware?
In the United States generally: Yes.
It is always legal to use force, including against police (see also here), if one reasonably believes it is necessary to prevent imminent (and unjustified) death or great bodily harm to oneself or another. (More detailed analysis of "defence of others" statutes here.)
Therefore, if a U.S. court finds:
Person A reasonably believed that the use of a taser on a particular individual B posed imminent threat of death or great bodily harm, and
Person A reasonably believed that the use of deadly force against B by a person C (who could be a police officer) was not justified, and
Person A took reasonable actions to prevent the use of a taser by C against B in that instant
then the court would have to find that the actions of person A were justified.
A search for "taser lawsuits" suggests a great deal of case law exists that might inform a person and court as to whether it is "reasonable" to conclude that the use of a taser against a particular person poses "an imminent threat of death or great bodily harm."
It's not entirely settled whether a third party can intervene in an arrest, and as commenters have mentioned, it'll depend on the jurisdiction and circumstances. This is an excerpt from a recent, thorough discussion of the issue, which includes some sample cases and statutes:
While the law concerning the use of deadly force against police officials in self-defense is relatively developed, the law regarding the third party use of force against police officials is slim. . . .
Section 35-41-3-2 of the Indiana Penal Code provides the clearest and, simultaneously the most controversial legal pronouncement regarding the use of force against public servants in defense of others. Section 35-41-3-2 provides, in essence, that an intervener may come to the aid of the perceived victim of unlawful police aggression and may, under certain circumstances, use deadly force to protect a third party, even a stranger, from excessive police force. . . .
The states are split in terms of the circumstances under which an intervener may use force in defending another from excessive police force. The case law on this issue is sparse. In 1997, the Supreme Court of Nevada, in Batson v. State, in an issue of first impression, held as a matter of state law that an intervener can use force against a police officer in defense of another party when the intervener witnessed the excessive use of police force. The court stated that:
[A] person may defend another only where that person has witnessed a police officer's unlawful and excessive use of force, and only where the individual being "rescued" is facing imminent and serious bodily harm at the hands of the police officer. Furthermore, an individual acting in defense of another against a police officer may only use that force reasonably necessary to remove the threat of imminent serious bodily harm to that other person.
. . . So in Nevada, an intervener's actions must meet four criteria to exonerate him or her under the state's defense of others doctrine:
The intervener must be a first hand witness to excessive police aggression;
The police aggression must threaten imminent and serious bodily harm to the defended party;
The intervener can only use the degree of force necessary to stop the threat of serious bodily harm;
The defended party must have in fact had the right to defend himself or herself.
The Supreme Court of Pennsylvania in Commonwealth v. French articulated a similar standard. . . .
Although most states have no statutory provisions specifically permitting the use of deadly force on public servants, the case law permits the force under certain circumstances. It is less clear under the law of these states the circumstances under which an intervener can use deadly force against a police official to aid a third party. . . .
Kindaka Sanders, A Reason to Resist: The Use of Deadly Force in Aiding Victims of Unlawful Police Aggression, 52 San Diego L. Rev. 695, 732-35 (2015) (footnotes omitted).