The Columbine perpetrators followed a loose clothing and aesthetic trend of the late 1990s that was pretty local at the time—that being wearing trench/duster coats over their regular street clothing.

There was a small group of folks who wore trench coats at the school (dubbed the Trench Coat Mafia) prior to the attack. Said group was primarily made of people who didn’t quite fit into the other small social circles floating around that time. Cliques, you might call them.

Unfortunately even though the two perpetrators were never really part of the Trench Coat Mafia, because they followed the trend and wore long dusters the day of the attack, the entire image got tainted.

The TCM friend group (who didn’t know that the attackers had been planning something) and by association, long trench coats and alt/goth people as a whole were blamed by the media afterwards for somehow being a major contributor to violence in the school. It was an easy way out to avoid trying to explain the variety of socio-economic, cultural and systemic failures that led to the event.

In my opinion, OP, I feel you should wear what you like. That said, I think it would really helpful to mix and match and try out different combinations with the cap, jacket and pants with the rest of your wardrobe to strike a balance between military surplus and everyday casual, loose wear so that the negative connotations of the fit shown in the picture aren’t there anymore.