Warning: this dream is super fucked up.
I've never been in any form of combat, nor do I enjoy watching military-esque movies, but I had a dream that I was in the Army and it was extremely vivid.
An officer was just tearing in to a group of cadets at attention about some super inferior issue. I remember being confused and kind of pissed that he was being so cruel. I see him pin-point a somewhat smaller cadet toward the back of the group and ask the inferior question to him directly. Yelling at him. Getting very close to his face and just yelling on and on about something so miniscule. But, the cadet stood his ground and continued to answer with utmost respect.
The officer then drew out his pistol and held it against the cadet's head, in a rage, and continued yelling the question. The group shifted in unease when they realized how much the situation had gotten out of hand, but they still stood firmly at attention. The cadet was obviously shaken and still didn't know how to respond. I remember seeing tears were welling up in his eyes. At this point he was trying to reason with the officer, staying respectful and following everything with "sir".
The officer had had enough and pulled the trigger. I remember the cadet slumping down to the ground and the rest of the group yelling and dispersing out of terror. The officer then went on a rampage and began shooting at everyone. It was a mass shooting. I remember running away from the tent where the officer and group had been and hearing bullets whizz by my head as I ran. I remember seeing only a huge expanse of desert in front of me, with nowhere to hide, and an incredible feeling of desolation and loss of hope.
After that, I woke up screaming. It was so vivid, like I was actually there at some point in my life. But I know I never have.
TL;DR: Had an extremely depressing dream nightmare about a mass shooting.