I was home in Chicago for Thanksgiving break during my sophomore year in college when I learned that Chaz Thrailkill, my former middle school classmate, had been arrested for murder. This was in 2011.

I was shocked. He was 19. We had attended the same selective-enrollment middle school in Chicago. I didn’t know him well, but he seemed like a cool person — he was a class clown, good-natured, with an easy laugh. In high school, he played wrestling and football. He joined the Marines right after graduation in 2010. He was scheduled to deploy to Afghanistan at the end of 2011. He never went.

On the night of November 23, 2011, Chaz dropped in at an off-campus house party near Northern Illinois University, in Dekalb, about 70 miles from Chicago. Late in the evening, he got into a fight with Steven Agee, a 22-year-old senior at the college. He shot him three times — twice in the back.

He pled guilty in 2014 and was sentenced to 30 years in prison for first-degree murder and attempted aggravated discharge of a firearm.

I found myself thinking about Chaz quite a bit in the years that followed. What decisions had he made that led him to so sharply alter the course of his life? Why did he have a gun in the first place, and how did a fight at a college party — hardly a unique experience — end with him pulling the trigger?

When I took an editorial fellowship at The Trace last year, I decided to try to find out. I contacted Chaz, and found him eager to talk. We spoke about the shooting, about his decision to take a plea deal, about the remorse he feels for taking someone’s life.

“I pray for [Steven’s] soul almost every night,” he said during one of our many phone conversations.

I contacted the victim’s family, too. Kimberly Agee, Steven’s mother, recalled feeling dismayed when she saw a young, fresh-faced Chaz at court hearings. She described him as “just a baby,” and wondered how he or anyone could pull out a gun and kill someone he didn’t know. She said it upset her to hear people describe this incident — in which her son was the only one to die — as a tragic end of two lives.

“Chaz didn’t lose his life,” she told me over the phone. “He only lost some of his freedom. His mother never had to walk behind her son’s casket like I did.”

Over the course of four months — in letters, phone conversations, and an in-person prison visit — Chaz reflected on the circumstances and split-second decisions that led him to shoot a man during a confrontation at an off-campus house party. Here is his story.

Steven R. Agee Jr. II, I would later learn, was the name of the man I shot and killed.

I was back home in Chicago on leave before my deployment in the Marines when my friend, a high school wrestling buddy studying at NIU, invited me to his college town to hang out for the week. I agreed to go.

While on leave, I often carried a gun. Being a rifleman in the Marine Corps, I felt like I needed to have a weapon with me almost everywhere I went. My gun was like an attachment of myself. We did shooting drills on a daily basis, almost religiously. The Marine creed is all about connecting to your weapon: “My rifle is my best friend. It is my life. I must master it as I must master my life.” I took the Marines and all of its lessons seriously, more seriously than I had taken anything before in my life.

My experiences growing up on the South side of Chicago also made me feel naked without my gun. Someone once drew a gun on me. The thought of that happening again, or that I could run into someone I once feuded with, or that I could be in wrong place at the wrong time, made me take my weapon with me almost every time I left the house.

So bringing my gun with me on my visit to my buddy’s school — and to this house party — just felt like second nature.

The shooting happened on the fifth night of my visit. We had been staying with my friend’s brother and his three roommates in a two-story, off-campus apartment. We were supposed to go back there to sleep after spending the night on campus playing cards and cracking jokes. Instead, we ended up at a house party. I didn’t want to go to another party. It felt like we had been partying every night, and I was over the whole scene.

It was around 1:30 a.m. when we arrived. There were 25 to 50 people in the apartment. My friend and I scanned the party for good-looking women before we retreated downstairs, where a few people were playing video games in a bedroom. While sitting in the corner, I heard a man and a woman arguing in another room. Then I heard a big “thump, thump, thump.” Then I saw a woman on the ground at the bottom of the staircase. I jumped up from my seat and approached the woman to see if she was okay. I saw a man, Agee, standing in the middle of the staircase. He stood at 6’1, several inches over me at 5’9. His post on the staircase made him seem even taller.

Everything from this point felt like it took place in a matter of seconds.

I began to question him about pushing the woman. He responded that he would do the same to me. He came down the stairs. I pulled out my gun. Initially, I figured I would just flash the gun and then he would back off. My friend tried to jump in between us at that point, and Agee pushed him out of the way.

He motioned toward my weapon. I remember him saying “I’ll take that shit from you.” I feared for my life. I shot him three times in front of everyone at the party that night — once in the arm as he went for my gun and twice in the back as he fell to the ground. I wish I could explain my reason for taking it to that extreme. Everything happened so fast. I didn’t take time to think it over. (Police reports and witness testimony generally corroborate Thrailkill’s version of events.)

I was supposed to be back home in Chicago for Thanksgiving dinner with my family the next day. I was supposed to finish up my leave in eight days and head back to base before leaving for my first deployment in Afghanistan.

I hid out for nearly nine hours in a nearby apartment hallway after the shooting. During that time, the news had quickly spread. I read tweets from students who wrote about not being able to enjoy their college experience because of shootings. This situation occurred just three years after a mass shooting on the NIU campus. I feel bad that my actions distressed so many people.

I thought I was doing a good thing by jumping into what I thought was a domestic violence situation. I didn’t predict how fast the situation would escalate after we exchanged words.

I could have ignored the altercation, like other partygoers, but I felt like I had to say something. This wasn’t my first time stepping in to defend a woman from a man. I saw women in my family get hurt by men, so I had a hard time just standing by when I saw stuff like that happen. While on weekend leave a few months before I shot Agee, me and my travel buddy witnessed a man fighting two women outside of a nightclub in Virginia Beach. We immediately jumped into the situation and fought the man. That time, I didn’t have a gun on me.

I made the decision to carry and I know that I must pay for my actions. I shot a stranger I had no prior encounters with three times after inserting myself into a conflict. He might have lived if I had stopped after the first shot, but in the heat of the moment, I didn’t consider that.

I took my plea deal thinking that at least I would have a chance to restart my life after prison. I will be nearly 40 years old when I get out. I won’t be able to go back to the Marines. I won’t have many job options as a felon. I always wanted to start a family, which doesn’t seem likely now. I realize that my life will never be the same. I want Agee’s family and friends to know that I am sorry.

My advice for other young people who might encounter similar situations is to think about the consequences of their actions. I know it is hard to think about the future, but every action has a reaction. I know that having a gun might seem like the best option, especially as a black man growing up in a city like Chicago, but question whether you really need it and why.

Are you ready to take on the consequences if you shoot someone during an altercation? Are you ready to deal with the aftermath of taking someone’s life?