After Charlie Kirk, the right-wing activist, was fatally shot during a political appearance on Wednesday, two thoughts occurred to Edward Padron, a 67-year-old locksmith in Brownsville, Texas. One was immediate. One was slower to rise.

A longtime conservative, Mr. Padron said he first assumed “a hate crime against a Republican” had just happened. But then he thought of other recent acts of political violence across the political spectrum, from the attempts last year on the president’s life to the fatal shootings in June of a Democratic lawmaker in Minnesota and her husband. It seemed to him as if some terrible disease was gripping the nation, with no cure on the horizon.

“This could happen to anybody in this country,” Mr. Padron said, speaking from his home near the Mexican border. “I think that people across the board are afraid.”

That anxiety echoed this week in interviews with Americans, including a group of voters that The New York Times has been following throughout the Trump presidency. No matter their politics, people said they were deeply unsettled after the killing of Mr. Kirk, who had built a national movement promoting right-wing politics on campuses like the one in Utah where his life ended.

Some of those interviewed had not heard of Mr. Kirk. Others felt strongly about him and his politics, for and against. But virtually all agreed that Mr. Kirk’s violent death seemed to confirm a deep fear that something is seriously wrong in this nation.

It was not just the gun violence. In some sense, that has become a daily tragedy, lamentable but unsurprising. As several people pointed out, there was also a school shooting in Colorado on Wednesday.

Instead, Mr. Kirk’s death at 31 symbolized for many the collapse of what they thought was a basic, common-sense, need-not-be-debated American value: that people expressing a political opinion should not be shot for it.

“There was someone on TV, and he kept saying that this was not who we are — that we are not one of those countries that shoots people over politics,” said Charles Phoenix, 62, a left-leaning artist based in the Los Angeles area.

“But it is who we are. We do shoot political leaders. We are that country.”

In interviews, people whipped through their own experiences and fears, trying to put a finger on just how things had become this ugly. They talked about friendships that dissolved into arguments, the infinite stream of disinformation, the knee-jerk name-calling, the calls for violence, the inability to see past partisan manipulation and calmly debate.

“Unfortunately, we are broken,” said Dave Abdallah, 60, a real estate agent in Dearborn, Mich.

Erwin McKone, 55, a salesman near Flint, Mich., who grudgingly voted for Donald Trump last November, was on the golf course when he received news alerts about the shooting. He did not know much about Mr. Kirk, but the news hit him like a gut punch. Fighting back tears, he said that the killing seemed to arise from an animus that was increasingly disconnected from facts, accountability and reason, to the point that he barely had the stomach to consume news these days.

“It seems like we’re totally living in insanity,” he said, “every moment of every day.”

In Marietta, Ga., Taylor Busch, 44, said the rancor over the Kirk shooting spilled into a Discord channel where he and other aficionados of the video game Destiny 2 regularly talk. Some of the members, he said, have known each other for 10 years.

“We’ve always kept it very dry, stayed away from political events,” said Mr. Busch, who voted for Kamala Harris last year. But after one of his friends posted a video of the killing, an argument erupted.

The gamers who supported Mr. Kirk bristled. They said, “‘Well, I like the things that he liked. I believed in things he believed in. You think I should be murdered?’” Mr. Busch recalled. “I’m like, no one’s saying that.”

Mr. Busch said that he was trying to make the point that Mr. Kirk “said these horrible things, and at the same time, it’s horrible that he died.”

Two people left the group, deeply offended.

Several people on the right reported a similar rush of fear and vulnerability.

In Utah, Bryce Youngquist, 43, a software salesman in Salt Lake City, said he was at a luncheon at his alma mater, Brigham Young University, when a host stood up and announced the news of the shooting.

Amid the gasps and the prayers, he said, his sadness for Mr. Kirk’s family was mixed with “kind of a terrifying feeling” that the shooting had, in some way, threatened him as a conservative.

“This feels like a turning point,” he said. “How are we going to move forward now?”

Liberals felt the threat, too.

Thien Doan, 36, a software engineer in Orange, Calif., said he had watched unhappily as his college-age nephews attended campus events where Mr. Kirk would be speaking. Mr. Doan disagreed with Mr. Kirk on nearly every issue and felt that many of his political statements — on gender, race, immigration — amounted to hate speech. But he was alarmed that Mr. Kirk had been killed.

Now he fears right-wing backlash. “Everything is just kind of worrying right now,” Mr. Doan said.

Some wondered whether Americans fully understood how rare it is to be able to peacefully exchange ideas — especially ones that provoke deep disagreement.

In Michigan, Mr. Abdallah, the real estate agent, worried that the shooting would chill free speech, worsen polarization and excite people with “the wrong mental direction” to violent action, as had happened so often in Lebanon, where he was born.

“I hope this type of thing does not discourage other people from going on campus and speaking,” he said, but the response to Mr. Kirk’s death made him fear for American discourse.

At the University of Wisconsin-River Falls, Emily Rose, a 19-year-old liberal, said that students on her politically mixed campus had already learned to hold back and largely discussed politics only with like-minded people.

“Sometimes when the conversation veers toward politics in a mixed space, things feel tense,” she said.

At the University of Wyoming, Charles Vaughters, 25, who served in the Marines before enrolling in college, worried that things could get much worse.

Although he disagreed with Mr. Kirk on some issues, he said that he appreciated the discourse. But, he asked, when people on the left compare the Trump administration to fascists and Nazis, how could political violence be surprising?

At the same time, he warned that the killing had turned Mr. Kirk into a modern-day martyr for the far right, especially for many college-age men who idolized him. When Mr. Kirk’s “American Comeback Tour” stopped at his red-state campus last spring, the auditorium overflowed with a sold-out audience of more than 1,800 people.

“A lot of young men are extremely angry about this,” he said.

In Lacombe, La., Clifford Eugene, 74, also worried about political rhetoric.

“Since the election of President Barack Obama, I feel like the country has gotten more and more divisive,” said Mr. Eugene, a retired bank examiner for the Treasury Department. “Now, you have leaders who demonize the other side and declare them enemies of the people.”

But what could be done? Mr. Eugene, a Democrat, was at a loss to name a remedy for this angry and entrenched nation.

“Political violence, in my opinion, is like water in a reservoir: A dam keeps it in place,” he said. “This moment feels like a dam buster. The gates are open and political violence is now part of our society.”

Johnny Kauffman, Ernesto Londoño, Juliet Macur, Laurel Rosenhall, Kurt Streeter and Sabrina Tavernise contributed reporting.