Devin Patrick Kelly fled the scene after opening fire at a church in Sutherland Springs.
Stephen Willeford explained how he was able to confront the killer.
He said: "I'm no hero - I am not.
"I think my God - my Lord - protected me and gave me the skills to do what needed to be done. And I just wish I could have got there faster."
The US Airforce has admitted it failed to pass on information about the Texas gunman.
Patrick Kelly's history of domestic assault should have prevented him from owning guns.
But the 26 year-old was able to buy a rifle just last year.
A memorial service for the victims of Sunday's church shooting was held on Monday.
