It's not so unreasonable. Perhaps the shooter has a very logical reason for having shot at the drone (and the neighbour's house, previously), but it seems to me very unlikely that there is any other reason than paranoia or recklessness. Having said that, i expect that a judge and/or mental health professional should evaluate this more carefully and with better access to facts than you or i on a forum somewhere—i'm just outing an opinion, not saying the shooter should indeed be treated/medicated/institutionalised.
> That kind of stigmatising use of language contributes to a culture where shooting and killing the mentally ill is common.
I actually think this is the wrong way around. I agree that we should recognise mental illness as a serious problem, and offer individuals who are suffering from it (and potentially endangering their entourage) professional help. I fail to see how suggesting that this shooter is mentally ill would contribute to the shooting and killing of mentally ill patients—on the contrary, it should lead to a more adapted response to this situation, instead of laughing it off or throwing a potentially mentally ill person in jail, which will be counter-productive, if anything. This shooter is not being stigmatised at all, as i see it: attention is being drawn to a potential sufferer who is not receiving (adequate) care.