Shoot the suicide bomber before a bigger boom. What if there was another person? Another thing? We can't know, they can't know. We know now, due to hindsight.

They are security. They secure scenes. They aren't paramedics.

I am not making pro cop statements here, but all the comments about "ohhh the cop arrived to a dangerous scene with a weapon drawn!" Is like saying "the garbage man picked up the garbage bin when he drove past my house!" Duh!