He was there from the early days of Hamas, rose through the ranks to lead the organization and equipped it for the deadliest assault on Israel in its history.

And now, Yahya Sinwar is dead, depriving the militant group of a ruthless, intelligent leader and raising questions about what direction its battered remnants will take in their fight against Israel.

Mr. Sinwar’s killing was a powerful blow to a violent organization that had already been gravely damaged by a year of brutal combat with Israel. Though he was only the latest senior leader to be killed since the war began, few experts expect Hamas to collapse. Still, the men’s elimination could cause a leadership vacuum and more chaos in its ranks.

Among the senior figures killed since January are Saleh al-Arouri, a key liaison with Iran and Hezbollah in Lebanon; Muhammad Deif, the shadowy head of Hamas’s military wing, who Israeli said was killed in July; and Ismael Haniyeh, who headed its political office in Qatar, making him central to cease-fire negotiations.

Mr. Sinwar was loathed by Israelis for starting the war and taking Israeli hostages, and resented by many Gazans who blamed him for the extensive suffering the conflict has brought to their lives. But he was revered by Hamas loyalists for helping plot the assault on Israel last October that left 1,200 people dead and 250 others dragged back to Gaza as hostages.

That made him an “iconic figure” among the group’s members, and a hard one to replace, said Fuad Khuffash, a Palestinian analyst close to Hamas.