Juan Severino Mallari was born on 22 September 1785 in San Nicolás, Pampanga (modern-day Macabebe) during the Spanish colonial era in the Philippines. He pursued theology at the University of Santo Tomas and was ordained in 1809. Early in his career, Mallari served as a coadjutor in parishes such as Gapan, Lubao, and Bacolor, before striving for full parish leadership in Orani, Mariveles, Lubao, and as a chaplain in Cavite—efforts that, at least initially, ended in failure.

Mallari eventually succeeded in 1816, becoming the parish priest of San Bartolome Church in Magalang, Pampanga, the first Filipino to hold that position in the province. Aside from his clerical work, Mallari had a creative streak—mastering calligraphy and decorating parish documents with ornate motifs like flowers and angels. He was acknowledged as the second Filipino calligraphic artist-priest, after Father Mariano Hipolito. 

From 1816 to 1826, while serving as the parish priest at San Bartolome Church in Magalang, Mallari’s convictions took a horrifying turn. He developed a delusional belief that his ailing mother was cursed and that killing people would somehow cure her. According to his confession, he murdered at least 57 individuals—likely stabbing them with a knife—and disposed of their bodies across the surrounding areas, which obscured any clear connection between the victims.

In 1826, Mallari fell ill. An attending priest who came to care for him discovered blood-stained personal items in Mallari’s home—an evident sign of his crimes. This led to his arrest, imprisonment, and subsequent detention for 14 years. 

During that time, some contemporary and later analysts, like Dr. Luciano Santiago, argued that Mallari’s mental instability should have warranted his placement in a mental health institution—such as the Hospicio de San José or San Lazaro Hospital—rather than prison.

Finally, in 1840, Mallari was executed by hanging under Spanish colonial laws.