The Parkland shooter admitted to a psychiatrist that he had planned a school massacre for over 5 years. He looked at previous shootings and studied what previous shooters had done. He learned to pie corners, he learned police response times, he learned to not let people get close.
He knew that he would have 20 minutes to carry out his plan without police interference.
Think about that while carrying in your day to day life. The shooter has a plan, he has the initiative, and you will be playing catchup.