How do you defend against something like this?

The details in the story of how Austin Hemmings was killed are horrible. He wasn't doing anything aggressive, he was just trying to intervene to save a woman from a man whom he must have realised was dangerous.

It just makes me wonder, how do you defend against an attack like this?

"I saw his left hand pull up his left side of his jumper as he did when facing me. I saw his right hand reach in and a movement of his right hand come down towards his waist and a really quick microsecond he thrusted really quickly and because I had seen the handle of the knife I realised he had stabbed Austin," Ms N said.

The man who killed Austin has pleaded guilty, so there was no trial.

Related link: Man guilty of 'good samaritan's' murder