re: People with Down's syndrome drinking in a bar

The way I read the OP’s post was similar to how the courts determine if someone is mentally competent and fit to stand trial. Is it legal to serve someone who is incompetent or mentally ill (not DS, because, as pointed out, they are neither). Would there be culpability if you served a known schizophrenic, off their meds and they murdered someone after they left the establishment?