Cheng Chieh was born on April 3, 1993, in the Banqiao District of New Taipei City, Taiwan. He grew up in a family with a younger brother and attended schools in Taipei. Cheng was known for being very focused on his studies. However, he had few friends during his early years and often became absorbed in violent online games. He also enjoyed writing horror stories.

As a child in elementary school, Cheng exhibited concerning behavior. He threatened to kill classmates over minor disputes. In junior high school, he carried a knife for a month, looking for an opportunity to harm his teacher. These actions reflected a troubling pattern that would follow him into his teenage years.

During high school, Cheng began to express his dark thoughts online. He wrote blog posts about his desire to kill and shared articles related to murders. After finishing high school, he enrolled in the Chung Cheng Institute of Technology in 2011, hoping to receive military training. However, he was expelled two years later. Following this, he transferred to Tunghai University.

While at Tunghai University, school officials were alarmed by Cheng's social media activities and offered him counseling. Despite this, no irregularities were found at that time. For a while, Cheng pursued studies in the Department of Power and Systems Engineering at the National Defense University. Here, he faced difficulties, failing to receive sufficient credits during his second semester and eventually dropping out. He then switched to the Department of Environmental Engineering at Tunghai University, but struggled there as well.

In 2014, Cheng attempted to transfer to the Chinese literature department during winter vacation, but he did not pass the exam. His family described him as having a lively personality during high school, which they noticed changed after he left the National Defense University. At the time of significant events in his life, Cheng lived with his parents and younger brother in Banqiao District.

On 21 May 2014, Cheng Chieh carried out a violent attack on a Taipei Metro train. The incident occurred around 16:25 local time while the train was traveling between Longshan Temple Station and Jiangzicui Station. This stretch of the subway had the longest gap between stations, lasting three to five minutes. During this time, Cheng used a 30 cm long fruit knife to attack passengers indiscriminately.

As he attacked, some passengers attempted to defend themselves and others by using umbrellas. They shouted at him in an attempt to distract him. When the train reached Jiangzicui Station, some passengers managed to subdue Cheng until police and metro staff arrived. Four people died as a result of the attacks, including a 26-year-old graduate student, two males aged 28 and 47, and a 62-year-old female. Additionally, 24 others were injured, with 10 sustaining critical injuries.

Cheng was apprehended and taken to a nearby police station. On 22 May, he was formally questioned by authorities. During his interrogation, he claimed he wanted to do something shocking and had planned the attack for many years. Although he expressed no remorse and stated he desired the death penalty, he insisted he was not guilty.

On 6 March 2015, Cheng was convicted of multiple charges, including four counts of murder and 22 counts of attempted murder. He was sentenced to death for each murder charge. His appeal to the Taiwan High Court was denied, and he was ordered to pay approximately NT$30 million in compensation to the victims' families.

Cheng's execution took place on 10 May 2016, shortly after his final appeal was rejected. He was given a last meal and then executed by lethal injection at a prison in Taiwan. The events surrounding Cheng Chieh led to significant public outcry, changes in police presence on transit systems, and discussions about capital punishment in Taiwan.