More than a month after the mass slaying at a care home for people with disabilities in Sagamihara, Kanagawa Prefecture, we are still struggling to understand what prompted the alleged killer to commit such a horrific act, while authorities explore why the crime was allowed to happen — despite all the signs that the 26-year-old suspect himself had provided about his intentions — and what can be done to stop a similar lapse from happening again.

Satoshi Uematsu, who surrendered to police in the early hours of July 26 after allegedly killing 19 residents and injuring 27 others at the facility where he had previously worked, has been quoted as telling investigators that he thought people with disabilities "had better disappear." In a letter he delivered to the official residence of the speaker of the Lower House in Tokyo in February, he wrote of his plan to "massacre 470 disabled people" and to "create a world where people with multiple disabilities can die in peace." He had also reportedly suggested that he had been influenced by the ideology of Nazi leader Adolf Hitler.

These reported remarks have led to speculation that Uematsu was driven to the stabbing spree out of a belief in eugenics. But such theorizing should not lead us to see the crime as an isolated case of someone with a peculiar way of thinking. The effort to prevent such hate crimes must explore the background to how the suspect, who was in daily contact with people with disabilities while he worked at the care home for more than three years until this spring, came to harbor such views.