Answer: Convince people to update their cultural norms and relationship models. Here's why.
Only a small proportion of all violence is an instrumental effort to get something, like someone else’s wallet, Alan Page Fiske and Tage Shakti Rai maintain in their 2014 book Virtuous Violence: Hurting and Killing to Create, Sustain, End, and Honor Social Relationships. Most violence, they say, springs from motivations within the perpetrator’s moral worldview, meaning that the perpetrator resorts to violence to constitute or regulate an important relationship “with a fully moral partner” especially as a kind of punishment or revenge. The perpetrator (and those around them) perceive it as a moral obligation to carry out the violence even if doing so triggers “guilt, shame, remorse, sadness, nausea, or horror” due to competing motives; overall, to the group, the violence “makes local sociocultural sense.”
Commonly, people understand their “setbacks, failures, illnesses, injuries, and deaths” to have been inflicted upon them by angry “deceased ancestors, spirits, or deities". This is just a supernaturally illustrated instance of the same principle. They believe the gods use violence to manage relationships, too.
Steven Pinker wrote the foreword, in which he claimed that his own prior book, The Better Angels of Our Nature: Why Violence Has Declined, included a similar claim that violent people believe they are acting for a higher good. He praised “Fiske’s theory of relational models [as] the best — indeed the only — overarching theory of social psychology.” This refers to an identification of four types of relationships — Fiske calls them "communal sharing" (CS), "authority ranking" (AR), "equality matching" (EM), and "market pricing" (MP) — any or all of which can be transgressed and thus lead to violence.
In particular, violence is used to uphold expected relationships based on who one is, such as establishing power dynamics based on race and gender (CS, AR), as well as on what one has done to earn one's treatment by others (EM, MP).
A 1996 paper by Bandura et al. in the Journal of Personality and Social Psychology proposed a scale to measure moral disengagement based on one's willingness to use violence and lies to protect one's in-group. Fiske and Rai, however, say that behavior sure looks like moral engagement to them. Even if your personal or cultural norms hold that “moral motives must be peaceful,” it doesn’t mean that people with violent motives aren’t engaging in moral reasoning of their own. Once you accept that the violence is morally motivated, it “makes no sense” to understand violence as dehumanizing its victim, since the perpetrator must assume the victim is human enough to bear guilt and to feel pain “in order for his punishment to have any moral meaning.” Perpetrators “fully appreciate that they are hurting fully human beings, and judge that it is right to hurt them.” The prefixes of dis-engagement and de-humanization are also confusing here because they imply "an original state of social relatedness…[and] moral engagement” that has been abandoned in the act of violence.
Violence is usually treated as “the essence of evil,” but this is a mistaken understanding. “Morality is about regulating social relationships, and violence is one way to regulate relationships.” Ordinary people “feel that it is morally right or even obligatory to be violent.” They do it “to create, conduct, protect, redress, terminate, or mourn social relationships with the victim or with others....to create, sustain, modulate, and repair the relationships that matter to them, to terminate relationships that become intolerable, or to mourn the loss of a partner.”
That is not to say that violence is (objectively) moral. It is only to say that real people perceive and reason it to be moral within whatever cultural view binds them. To argue that violence is truly immoral (as seen from some outside perspective) or that the violent person somehow misunderstands their own cultural norms is not the project of Fiske and Rai's book.
The authors present a recipe for "the only way to reduce morally motivated violence": Bolster relationship norms that support nonviolence and prohibit violence, grow networks of relationships that uphold these norms with complete clarity, and build awareness and consensus about these norms and relationship networks so that everyone knows that everyone else agrees on them. This is "what cultural change consists of: consensual transformation of preos and metarelational models." "Preos" and "metarelational models" are their funny words for cultural norms and complex relationships between multiple people. It just might work.