There's more to this story.
This guy was probably a shitbird that got into trouble multiple times, couldn't get promoted (or possibly had some stripes taken away), but didn't do anything (or get CAUGHT doing something) bad enough to get a less than honorable discharge.
It should be thoroughly investigated, but it sounds like this guy brought dishonor upon the Corps (not to mention almost KILLING two people, or more, if the downed chopper had hit someone on the ground) and should be put away for a long, long time.
This guy was most likely no sniper.
From the word marksman he qualified at the lowest level possible. To qualify as marksman is like getting a C grade