The gunman who killed at least 49 people in a shooting rampage at an Orlando nightclub has been identified as 29-year-old Omar Mateen. Here is what we know about him so far. (Deirdra O'Regan/The Washington Post)

Whether the words come from politicians, pundits or the general public, they've all been been heard before -- in Oklahoma City and Charleston, S.C., in Sandy Hook, Conn., and San Bernardino, Calif. And now in Orlando:

"Hatred," "bigotry," a "twisted ideology," a "poisoned psychology."

But in the grim aftermath of Sunday's carnage at a gay nightclub in Florida, with at least 49 dead and 53 wounded, what can science tell us about the motivations of a mass murderer?

Reports suggesting that gunman Omar Mateen was perhaps struggling with his own sexuality are an important clue.  On Sunday, the killer's father said his son had recently become enraged when he saw two men kissing. By Monday, there were confirmed accounts that Mateen had frequently visited the Pulse nightclub and used gay dating apps.

"Hate of other people is really displaced hate of oneself," said social psychologist Arie Kruglanski, a professor at the University of Maryland. "It's about the loss of a sense of significance, that one is important, and that can happen because of personal failure or being part of a discriminated minority or being bullied. There are many different ways people can feel insignificant."

The neurological correlates of hate have not been determined -- not surprisingly, the emotion is difficult to study in a laboratory -- but scientists know where in the brain moral disgust arises and that the motor cortex is among the regions activated when a person feels aggression. They also know that our brain is constantly surveying our environment, always on guard as it assesses whether someone or something nearby is a friend or foe.

Ultimately, feelings of self-hate motivate people to restore their sense of significance through action. And the fastest, most efficient, most powerful action, said Kruglanski, is "the most primitive, primordial act a human being [can take] -- showing one's power over other human beings."

In other words: violence.

The ideologies of the Islamic State -- to which Mateen declared allegiance in a 911 call from within the nightclub -- appear to have served his need. According to anthropologist Scott Atran, those ideologies essentially told him to "stand up, avenge your dishonor."

Someone like Mateen could have been particularly susceptible to such a message. While it doesn't appear he had any formal contact with terrorist groups, he clearly identified with them, as well as with others he viewed as victimized groups, including those from his parents' native Afghanistan.

"We heard him talking to 911 saying the reason why he’s doing this is because he wants Americans to stop bombing his country," said Patience Carter, a survivor of Sunday's shootings.

Carter, who was among several patrons Mateen held hostage after they'd tried to take refuge in a club bathroom, told a news conference Tuesday that Mateen asked if there were any African Americans in the bathroom. When one man answered yes, "the gunman responded back to him saying, 'You know, I don't have a problem with black people. This is about my country. You guys have suffered enough.' "

Harvard psychologist Mina Cikara understands that kind of thought process. "The idea is when people perform an act of hatred on behalf of a group, it’s not as much about hate of the other group as reinforcing values of their own group," she said.

When viewed through an evolutionary lens, motivations based on group identification make sense because we're fundamentally communal beings, scientists say. It takes the human brain mere milliseconds to distinguish the faces of family members from those of strangers, according to a 2013 study in the Journal of Experimental Social Psychology. Psychologists call this tendency "in-group favoritism," and it's seen in both young and old. Young children, however, add something else, too: "out-group hate," or a desire to hurt those who are different.

This other characteristic usually disappears by adulthood. Why it doesn't for some is not fully known.

In a series of experiments several years ago, psychologist Adam Waytz of Northwestern University's Kellogg School of Management showed that the more people are socially connected to a group, the more likely they are to believe others outside the group are less intelligent, even less human.

A colleague, social psychologist Nour Kteily, says feeling dehumanized and alienated from mainstream society "can underlie actions like in Orlando," where one person goes on a murderous rampage.

"I don’t know what drove this particular individual," Kteily said Monday. "But these types of [negative] depictions of Muslims have been increasing with candidates like Donald Trump. Disparaging minority groups and likening them to criminals and animals -- people are cognizant of these."

Taking action to ameliorate a perceived injustice actually has a neural benefit, scientists say, because revenge triggers the same pleasure centers in the brain as happiness.

We may never know what really was going on inside the mind of Omar Mateen when he calmly opened fire inside the crowded Pulse nightclub shortly after 2 a.m. Sunday. But trying to place blame either on guns or a terrorist ideology, as many Americans are doing, is "a false choice," according to Kruglanski.

"They’re not mutually exclusive," he explained. "Ideology removes the moral obstacle. ... What’s difficult to do is to prevent people from experiencing failure or being dishonored."