Deyan Valentinov Deyanov, a Bulgarian national born around 1982, led a transient lifestyle across Europe, including stints in the UK, Cyprus, and Spain. He struggled with drug addiction, notably crack cocaine and LSD, and exhibited signs of severe mental illness, specifically paranoid schizophrenia. In 2010, while in Wales, he was admitted twice to the Ablett Psychiatric Unit at Glan Clwyd Hospital. Despite displaying delusional behavior, including claims of being the Messiah and expressing violent intentions, he was discharged after being misdiagnosed as malingering. This misdiagnosis was later criticized in an independent review, highlighting significant shortcomings in his mental health care.

On 13 May 2011, in Los Cristianos, Tenerife, Deyanov attacked 60-year-old Jennifer Mills-Westley, a British retiree, inside a Chinese-owned general store. Without provocation, he stabbed her in the neck multiple times with a 22cm knife, then decapitated her. He carried her severed head into the street, reportedly shouting phrases like "This is my treasure" and "God is on Earth." A bystander subdued him until police arrived.

At his trial in February 2013, Deyanov pleaded not guilty by reason of insanity. The court acknowledged his schizophrenia but found him guilty of murder, sentencing him to 20 years in a secure psychiatric facility. The case sparked widespread criticism of mental health care systems in both the UK and Spain, with calls for systemic reforms to prevent similar tragedies.