The senseless, brutal and futile murder of PC Keith Palmer in the grounds of Parliament, and of the people walking on the nearby Westminster Bridge, has shown us the very worst of humanity ... and the very best.
I was in the capital on Wednesday for a meeting at the Ministry of Justice, not far from the scene of this terrible attack. The sound of sirens filled the air, and text messages asked if I was safe. I was, but tragically the people sightseeing on the bridge and PC Palmer - who was heroically protecting MPs - were not.
It has been so moving to hear bobbies in the capital speaking of members of the public coming up to thank them for what they do. As one said: "It's not often we get compliments like that".
In those first moments after Khalid Masood's hire car mowed down the pedestrians, we saw ordinary people doing what they could for the victims before the doctors and paramedics were on the scene. And we saw an MP trying, through mouth-to-mouth resuscitation, to save the life of PC Palmer.
In Trafalgar Square we saw people of many faiths and none, standing together in a candlelit vigil to remember those who lost their lives. People there spoke of their wish to express solidarity - and their desire for peace.
We also saw the next day a determination that life must go on, and that, as the Mayor of London put it: "Londoners will never be cowed by terrorism".
We don't yet know why Khalid Masood chose to turn his vehicle into a lethal weapon. Some say that the motive of such attackers is to split communities, to turn people against one another. To sow hatred.
All of us must do what we can to ensure that hate does not triumph. Members of the Muslim community, who are just as shocked and appalled as those of us who do not share their faith, will be feeling anxious. Leaders of their community have condemned the attack and offered prayers for the victims.
As Commissioner I chair a hate crime group which brings together people from a wide range of backgrounds, representing minority communities in Wiltshire and Swindon. We all share a commitment to standing together to defeat prejudice.
Today, as a gesture of solidarity, I went to the Trowbridge Mosque to talk to worshippers after Friday prayers. I can report that they were unanimous in rejecting the jihadists and their perverted view of Islam.