Passengers described chaos as Cheng moved through the train car, slashing and stabbing indiscriminately. In the confined space of a metro carriage, there was no clear escape route while the train was in motion. Yet people fought back. A group of passengers used umbrellas to create distance between themselves and the attacker. Others taunted him loudly, trying to distract him from pursuing victims who had fallen. When the doors finally opened at Jiangzicui Station, passengers, police, and metro staff subdued Cheng after a brief standoff. He was taken to the nearby Haishan police station. The four people who died were Chang Cheng-han, a 26-year-old graduate student; Hsieh Ching-yun, 28; Pan Pi-chu, 47; and Lee Tsui-yun, 62. Ten of the 24 wounded were in critical condition.

Cheng Chieh had been thinking about violence for most of his life. In elementary school, he threatened to kill classmates over minor disagreements. In junior high, he carried a knife for a month looking for an opportunity to stab his teacher. In high school, he blogged about murder. He enrolled at the Chung Cheng Institute of Technology in 2011 hoping for military training but was expelled two years later for academic failure. He transferred to Tunghai University, where officials noticed his disturbing social media posts and offered counseling that revealed nothing actionable. During police questioning, Cheng said he had originally planned to attack after graduation but moved the date forward because he had no classes on May 21 and had "grown tired of living." He expressed no remorse, stated he would have killed his own parents had they been aboard, and said he wanted the death penalty.

The attack sent shockwaves through a city that prided itself on one of the safest metro systems in Asia. President Ma Ying-jeou condemned the attack and ordered the National Police Agency to investigate. Within days, police officers were permanently deployed at all 109 stations in the Taipei MRT system. The Taipei Rapid Transit Corporation covered victims' medical expenses and set aside NT$4 million for their families. Ridership dropped by 945,000 within ten days, costing the system NT$20.61 million. Tunghai University released a statement calling Cheng "our family" and urging students to care for one another. The incident also sparked a charged public debate about the death penalty, with a Facebook group supporting Cheng's execution quickly accumulating over 32,000 followers. On May 27, Cheng's parents publicly apologized at Jiangzicui Station and asked for a swift death sentence for their son.

Cheng was indicted on four counts of murder and 22 counts of attempted murder. The New Taipei District Court sentenced him to death plus 144 years imprisonment and ordered him to pay approximately NT$30 million to victims. The Taiwan High Court upheld the sentence and later added another NT$61.39 million in damages owed to victims and families. Cheng became the first death-row defendant to appear before Taiwan's Supreme Court in person when his final appeal was heard in April 2016. The court upheld all previous rulings on April 22. Eighteen days later, Justice Minister Luo Ying-shay signed the execution order. That evening, Cheng ate his last meal, a biandang of stewed pork, rice, and vegetables. He was given general anesthesia and executed at 8:47 PM on May 10, 2016. The gap between his final verdict and execution was the shortest in Taiwanese judicial history. The trainset where the attack occurred was disinfected, renumbered from 117/118 to 175/176, and returned to service in May 2015.