I’m fairly certain a history of being tried in court because you’re a trigger happy murderer also likely disqualifies you from joining the armed forces. That’s too much of a ticking pipe bomb even for them.
The murder happened in August of 2020 and this says the test was taken in January 2020. This is seven months before then so I don’t think that played a role.
It does show he wanted to join the military, presumably because he had a fascination with killing people. Maybe being rejected directly led to him finding a way to kill someone without joining the military.