Witnessing a friend dying right before his eyes, for 18-year-old Pfc. Brendan Bialy that was hard. The Colorado teen at that time searched for a way to overcome the tragedy—becoming a Marine.

Bialy was at the scene when two shooters appeared at STEM School outside Denver on May 7. Eight students were injured and Bialy’s friend Kendrick Castillo, 18, was killed. Bialy acted when he saw Castillo charge the gunman, according to FOX NEWS

At a news conference after the shooting, Bialy said he tackled the gunman and fought back out of pure instinct.

Bialy had signed up to be a Marine under the service’s delayed entry program and went to boot camp after graduating. After months of training, on Sept. 20, he graduated from the Marine Corps Recruit Training, Stars & Stripes reported.

“My thoughts on becoming a Marine were nothing but reinforced after the shooting,” he said in a video statement last week. “What I saw that day was complete and total malevolence, bad overcome by good.”

During training, he earned a meritorious promotion to private first class.

“I don’t think recruit training changed Bialy,” said his drill instructor, Staff Sgt. Marcus Chestnut, according to the news outlet. “He is who he was when he first got here. I think we just gave him some additional attributes that made him a stronger man and a basically trained Marine.”

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