I hope they sue the pants off who ever had a policy for this punk to carry around an assault rifle.
They can try, but success will be questionable. The general policy of letting rural sheriffs deputies take their duty weapons home is not really unreasonable for a small department with a lot of territory to cover.
Now the policy of hiring a 20 year old is somewhat questionable, especially if the training course is the common "community college" type of short course, with minimal field training. In a larger department, they'd have the "kid", even out of much more extensive training program, ride along with a veteran for a good long while. I doubt this little department can afford to do that. That being the case, they'd be better served by hiring veteran officers, either military veterans, likely to be a bit older and maybe quite a bit more mature (or at leas t you could base your hiring decision on a track record from their military service) or veterans of other larger police agencies.
I'm sure in this case the sheriff, and the Chief of police where he was a part timer, *knew* the kid for quite a while, as everyone in these small communities knows everyone else. Even if the Sheriff didn't know him, or didn't know him well, the Chief would have, and the Sheriff would have consulted with him before the hire..at least I'd hope so.