From what I read, the shooter was inside the restaurant and the two men confronted as he came out, they ordered him to put his gun down and then shot him when he fired at them.
https://www.usatoday.com/story/news...ho-shot-oklahoma-restaurant-diners/643726002/
https://www.usatoday.com/story/news...ho-shot-oklahoma-restaurant-diners/643726002/