"Amongst the various consequences of this (including an email from my head of department which, frankly, scared me because I was so new to it all), was getting an 'explanation' email from the student in which he told me he had, in his previous job, been in the army and therefore knew all about guns and killing people."
He'd clearly missed the bit of Army training which covered doing as you are bloody well told, @LRDtheFeministDragon!!
I agree with the posters who have said that these students are young adults, not children, and it does them no favours to expect less of them, in terms of behaviour.
At 18, I was a student nurse - back in the days when we did most of our training on the wards, before the advent of degree nursing - so when my classmates and I were the age of these students who can't turn up on time to a lecture, pay proper attention or do the work they are supposed to, we were responsible for the care of some seriously ill patients. We were not solely responsible - there were senior students and trained staff on the wards, but we were expected to do the jobs assigned to us properly, and we would have been hauled over the coals by the dragon of a ward Sister, if we had not turned up on time!!
By the time we reached the third year of training, we were considered sufficiently well trained and responsible to be left in charge of a ward at night, whilst the trained staff were on a break, and to do the drug round, and complicated dressings and treatments.