Weeks after Hamas attacked southern Israel, Sharone Lifschitz stood in the charred ruins of her parents’ home in the Nir Oz kibbutz and listened to the bombs falling on the nearby Gaza Strip.

Five bullet holes scarred the door of the safe room where her mother and father—both longtime peace activists and founding members of the kibbutz—sheltered for hours until Hamas militants broke through on Oct. 7 and took them hostage. Her mother was released weeks later; her 83-year-old father is still being held in Gaza.

“How do you keep humanity?” said Lifschitz, who is 52. “How do you not wish destruction on their children?”

The Oct. 7 attacks that Israeli authorities say killed more than 1,200 people, mostly civilians, are prompting a reckoning in parts of southern Israel that bore the brunt of the violence, and that for decades have been home to scores of activists who sought to promote coexistence and greater rights for Palestinians.

The region, home to many collective communities known as kibbutzim, has also been far more supportive of pursuing negotiations for a two-state solution with Palestinians than the rest of Israeli society, which has moved sharply to the right since the failure of peace talks more than two decades ago.

In Nir Oz, Be’eri and Kfar Aza—three of the kibbutzim that were among the hardest hit in the Hamas attacks—right-wing parties garnered on average less than 10% of the vote in last year’s elections, which resulted in what analysts consider the most right-wing, religious and ultranationalist government in Israel’s history. Among Israelis who identified as left-wing, 83% were in favor of a two-state solution, in contrast with 16% on the right, according to a recent poll.

Overall, support for peace negotiations with the Palestinian Authority among Israeli Jews fell from 47.6% in September to 24.5% in a survey conducted in late October by Tel Aviv University’s Peace Index. Support for a two-state solution among Jewish Israelis dropped from 37.5% in September to 28.6% in the aftermath of the attacks, the poll said.

“The kibbutzim were bastions of the left. You’ll find in the kibbutzim people who went out of their way to cooperate with Arabs,” said Benny Morris, a historian of the Israeli-Palestinian conflict. “I fear that Oct. 7, with the mass killing of civilians, has further undermined the left in Israel.”

For years, the left in Israel drove the peace process. The Labor Party, whose roots go to the state’s founding, led by Prime Minister Yitzhak Rabin, signed the 1993 Oslo Accords that were seen as laying the groundwork for a future Palestinian state.

After the collapse of peace talks, Palestinians launched a yearslong bloody uprising known as the Second Intifada that left many previously pro-peace Israelis disillusioned. Support for a two-state solution fell and right-wing political parties rose to power, and have continued to dominate Israeli politics.

Now, activists in southern Israel who have long said that peaceful coexistence with Palestinians was possible are being forced to rethink those beliefs in the aftermath of the attacks, which also resulted in some 240 people being taken hostage. The Israeli military campaign has so far killed more than 15,500 Palestinians, mostly women and children, according to the health authorities in Hamas-controlled Gaza. The number doesn’t distinguish between civilians and combatants.

“I feel such as I’m fighting to stay human and to not change, but it isn’t easy,” said Yael Noy, the head of Road to Recovery, an organization that helps Palestinians in Gaza and the West Bank access medical care in Israel.

Some of Hamas’s victims were involved in fostering ties with Palestinians, including veteran Canadian-Israeli activist Vivian Silver, whose remains were identified last week. Silver, 74, was a co-founder of Women Wage Peace, which promotes peace-building efforts, and worked as a volunteer for Noy’s group, driving Palestinians from the Gaza Strip to hospitals in Israel.

A person arranges wreaths under Israeli flag, as friends and family mourn Vivian Silver, 74, a Canadian-born peace activist from Kibbutz Beeri who was killed in the deadly October 7 attack by Palestinian Islamist group Hamas from the Gaza Strip, at a memorial service, at Kibbutz Gezer, Israel November 16, 2023. REUTERS/Ronen Zvulun

Three other volunteers for Road to Recovery were also among the victims of the attacks, and several others were taken hostage. Noy said that since the attacks, she has faced anger from neighbors on her parents’ kibbutz who say she should abandon her peace-building work, and some of her volunteers have yet to return to their work.

Nonetheless, Road to Recovery, whose founder’s brother was killed by Hamas three decades ago, resumed transporting Palestinians from the occupied West Bank to hospitals in Israel the day after the attacks, Noy said. It is no longer possible to collect patients from Gaza because Israel has sealed off the enclave.

“It is a big struggle inside myself to keep on doing it and maintain hope,” she said.

Activists in Israel have found partners in the Palestinian territories who have similarly defied a hardening of their society to pursue peace.

But the current war will further shrink the space for activism on both sides, said Khalil Shikaki, director of the Palestinian Center for Policy and Survey Research in Ramallah, given the civilian death toll in Gaza.

“Only the very committed, those who are willing to risk everything, will be willing to continue to do so again,” Shikaki said.

Avi Dabush has also returned to his activism. About two weeks after the attacks, Dabush drove with colleagues from Rabbis for Human Rights, which he leads, to the West Bank to act as human shields for Palestinian farmers who face threats from Israeli settlers.

“What happened was shocking and terrible,” Dabush said about the Hamas attacks. “But my values didn’t change. If we want to be in the Middle East, we need to push those values ahead, the values of peace and equality and justice.”

At the same time, the trauma of Oct. 7 has changed how he views Hamas, which the U.S. has designated a terrorist organization. That morning, Dabush hid for eight hours in a shelter in Kibbutz Nirim, near the Gaza Strip. Five fellow residents were killed.

Before Oct. 7, Dabush was one of a small minority on the left who said Israel should engage with Hamas in peace talks, but that has changed, and he says the group needs to be eradicated. “Right now I can’t see any way that we, the communities there and myself and my family, can live with Hamas on the border,” he said.

Sharone Lifschitz’s father, Oded, is a journalist and longtime advocate of Palestinian rights who had driven Palestinians to hospitals in Israel as a volunteer for Road to Recovery. Lifschitz said her mother, Yocheved, was equally committed to peace.

When Hamas released her mother in early November, the frail 84-year-old turned to shake hands with one of her captors, saying “shalom”—meaning peace. The gesture provoked an outcry among Israelis who said it burnished Hamas’s image. Others noted that her husband was still a captive.

But for Lifschitz, it was a sign that her mother’s perspective remained fundamentally the same. “Her worldview hasn’t changed after her experience—on the contrary,” she said.

Lifschitz, who lives in London, is now campaigning for her father’s release and seeking a way forward for peace advocates such as herself. “It is such a dark moment that maybe it could be the catalyst for something else that is better,” she said. “What is the alternative?”

—Anat Peled contributed to this article.

Write to Sune Engel Rasmussen at sune.rasmussen@wsj.com and Isabel Coles at isabel.coles@wsj.com