Most people with a gun trained on them would be compliant and maybe for a moment it was thought he was, enough for an officer presumably to drag the member of public out of the way. The terrorist then choose to get up against the instructions being directed at him to stay down, he didn't and a skilled firearms officer judged it to be on balance a clear shot to neutralise the threat. We don't know at what point the vest was identified, whether the restrainer called it out or it was spotted whilst the restrainer was being pulled away. Whoever the firearms officer was made the call as it unfolded based on several bits of info, at worst if he wasn't a terrorist they knew he'd just stabbed 5 people so at worst it was a reasonable call that this wasn't time for a straw poll on what to do next.