One could argue that an off-duty cop is not a civilian but he was there as a civilian and not serving as a LEO. Furthermore, he was related to the shooting victim so consider that a family member makes the save of others. Consider, too, the wealth of restrictions on concealed carry that make it more likely a random person in the crowd who is carrying may be a LEO. That is a pretty high percentage in some jurisdictions.
A case in point. The Works girls were killed in the parking lot; Jeanne Assam put the gunman down when he enteredthe building.
Main article: 2007 Colorado YWAM and New Life shootings
On December 9, 2007, a gunman opened fire in the New Life Church, striking four people and killing two, sisters Rachel and Stephanie Works. Jeanne Assam, a church security volunteer, shot and wounded the gunman who then killed himself.
The gunman, identified as Matthew Murray age 24, was formerly a missionary-in-training with Youth With A Mission and was from a devout Christian family.
About the shooting, Jeanne Assam said, “I just prayed for the Holy Spirit to guide me. I said, ‘Holy Spirit, please be with me...’ My hands werent even shaking.”  Assam’s shots were non-fatal. The Coroners report identified that the fatal shot was self-inflicted. Police found a letter from the shooter addressed “To God”.
At a congregational recovery meeting three days after the shooting, Boyd told parishioners they “will not be governed by fear.” Boyd appeared with Jeanne Assam on a host of media appearances following the tragedy. On April 17, 2008, the Colorado State Senate honored Jeanne Assam passing a resolution calling her a “true hero”.