Even in a region where violence has become all too commonplace, the killing of four Shiite men in Egypt last weekend seemed particularly vicious. According to news reports, a cheering Sunni Muslim mob armed with clubs, swords and machetes raided a house in a Cairo suburb where about 30 people were marking a religious festival and beat, stabbed and lynched the four men. Video footage showed the victims’ bodies, bloodied and motionless, being dragged through the streets. Among those killed was a prominent Shiite cleric, Hassan Shehata.

The incident illustrates a pernicious sectarianism that was largely repressed by pre-Arab Spring dictators but that now threatens Egypt and much of the Arab world. If left unchecked by newly elected leaders who either exploit simmering historical animosities or refuse to address them constructively, divisions will worsen between Sunnis and Shiites or between Muslims and other minorities, like Christians, ensuring prolonged regional turmoil.

In Egypt, President Mohamed Morsi and the Muslim Brotherhood, the Sunni Islamist party from which he hails, have failed to unite the overwhelmingly Sunni country and its Christian and Shiite minorities around a centrist agenda in the post-Mubarak era. Instead, they have solidified ties with Salafist hard-liners in the Islamist camp; derided opponents, including many secularists, as “enemies of Egypt”; and demonized Shiite and Coptic Christian minorities.

The mob attack came after months of anti-Shiite hate speech; the week before the incident, Mr. Morsi appeared on stage at an event with hard-line Sunni clerics who denounced Shiites as “filthy.” Tensions over Mr. Morsi’s rule have been running strong for some time. On Friday, at least three people were killed and hundreds were injured in protests, and many fear violence during protest rallies set for June 30, the first anniversary of his inauguration. The army has warned it may intervene if things get out of control. Mr. Morsi’s speech to the nation on Wednesday offered little to appease critics demanding his resignation.