And it would be traumatic for those who witnessed the onslaught unfold, and possibly saw police officers shooting the Isis-influenced extremist dead.
There are plenty of other people who would have found the incident hard to deal with. They include the police who shot the attacker, having kept him under watch for 53 days hoping it wouldn't come to this.
Those directly affected by the 2019 Christchurch mosque shootings would have suddenly been faced with painful memories. Members of the communities to which the offender belonged may also have worried they would be targeted for undeserved abuse.
As the Prime Minister put it on Friday: "What happened today was despicable, it was hateful, it was wrong. It was carried out by an individual, not a faith, not a culture, not an ethnicity, but an individual person who is gripped by ideology that is not supported here by anyone. He alone carries the responsibility for these acts, let that be where the judgment falls."
The 'lone-wolf' attacker had been in the country since 2011, arriving on a student visa from Sri Lanka, and granted refugee status in 2013. He spent three years in jail after being arrested in May 2017, suspected of heading to Syria and charged with possessing hunting knives and objectionable material.
Attempts to charge him with plotting an attack or to deport him failed. He was released last July and has since been under surveillance. GPS tracking was sought but denied by the courts.
The legal issues surrounding the Isis sympathiser and assessment of what went wrong defy easy answers, even as political leaders aim to rush through tougher terror laws.
A bill introduced in April would make it a criminal offence to plan or prepare for a terrorist attack. But experts say it may not have made a difference in this case.
People acting on their own have the advantage of surprise and can quickly inflict damage. They may also act impulsively on thoughts that have long been baking rather than be more obviously deliberate.
Prevention in some way is clearly better than ending an attack that's underway. Yet there are also questions over targeting a suspect for appearing to want to commit an act.
Could such laws be misused in future? Can links be automatically drawn between any suspect showing interest in something hateful and physically acting on it? The internet is full of toxic views and people willing to support them.
Strong emotions at these times are understandable but responses to horrific acts still need careful consideration to prevent them happening again.