He probably didn’t realize he could quit basic, thought he’d be for sure spending the next four years at least in the army, and then decided on a really dumb plan to try and escape, because he’s an idiot.
The follow up is that they aren’t allowed to carry guns at Fort Jackson.
https://www.armytimes.com/news/your-army/2021/05/14/trainees-not-getting-rifles-at-fort-jackson-after-one-hijacked-a-school-bus/
Which is a pretty intrinsic part of basic.