At what point does someone decide to become a mass killer? This is the confronting question Nick O’Malley,Tim Barlass and Patrick Begley set out to answer as theypieced together the life of Brenton Tarrant,the 28-year-old Australian who killed 51 and injured 49 others in Christchurch on March 15 this year.

The answer is vitally important to efforts to protect societies from the likes of Tarrant,a white supremacist who bought the line that members of the Muslim faith are"invading"the West.

Months of meticulous research led this trio of reporters to conclude that an odd loner who grew up in small-town NSW was radicalised when he immersed himself in an extreme-right subculture that spans both the online and real worlds. In this world,fellow travellers share their vile prejudices freely and joke about things the rest of us find abhorrent – like murder.

Some will say we should not be writing about this killer. We understand that view,and do not publish lightly. If knowledge is power,however – and we think it is – then exploring how Tarrant was radicalised might just help in the fight against the warped ideology he represents.