This is squarely the Mind-Body problem. A number of points here, though.
"Chemical Imbalances" is a highly misunderstood idea. We have never demonstrated a causation between nerve function and conscious experience or volition. What we did notice, is that if given Reserpine, a drug that depletes dopamine stores, people subjectively report feeling depressed. Subsequently this finding was extended to use of anti-depressants such as the Selective Serotonin or Noradrenaline reuptake inhibitors (SSRIs, SNRIs) or Monamine Oxidase inhibitors (MAOIs). In medical studies they seem to help those with depression, but no causation has been established. It is a tentative theory, as measured levels don't seem to support this. There are other theories in Psychiatry, but this is the go to one to tell patients, as we inevitably prescribe some antidepressant.
That this is really even a true effect has not been established. Kitsch did a whole series of meta-analyses, taking into account secondary placebo effect where an active substance is tested vs an inert placebo so that people can guess which is which, and found antidepressants no better than placebo except for Sertraline. By Evidence-Based means we still prescribe them by the thinnest of margins, but the 'chemical imbalance' theory for depression is skating on thin ice indeed. One should think this, as very different drugs, that act on different pathways, all show similar effect - which suggests similar mode of action, and thus makes their different pathways of action perplexing. This was what made Kitsch hypothesise a common pathway of action via extended placebo effect. So, not discarded, but very far from being an established fact.
You should remember that if fully explicable organically, then it is Neurology. If able to be shown to be mentally-derived, then Psychology. Psychiatry mans the vast nomansland between these, where elements of both are presumed present. So for instance, some of the most effective treatments for Depression are things like Cognitive-Behaviour Therapy, which is essentially just counselling to retrain your brain. With Psychiatric illness there is a chicken or the egg thing going on, as though physical changes in the brain occur, we cannot assert this is the cause of the syndromic picture or the consequence of mental ideation - especially as psychotherapy and neural plasticity alters these changes.
Don't overestimate what we know. We have subjective reports what people are thinking, which we couple to fMRI pictures, but this doesn't show material causation for those thoughts unless you assume a material origin solely beforehand. We have what are called Neural Correlates of Consciousness, so we can show what areas are vaguely associated with decision making, but we haven't shown those areas as causative. They may just be a consequence thereof, which studies on muscle relaxants and EEG and fMRI studies equivocally support.
So quite frankly, you cannot blame decisions made on deranged neural activity straight. You can have delusions or hallucinations resulting in decisions made, but those are decisions based on erroneous perceptions, not decisions themselves that can be shown to be non-volitional.
Similarly with substance use. I cannot give you a substance and mould you to my will. Certain substances, like alcohol or Ketamine, disinhibit you or dissociate your actions. They increase suggestibility, but they cannot force you to do something against your will. They can remove qualms say, or suppress conscience, but not make you kill someone if you are a conscientious objector, say. Movies really oversell these things.
So the way I see it, some people receive erroneous data from their senses or brains, but their conscious actions, their volition, remains. If you really think everyone is trying to kill you, then it seems perfectly reasonable to defend yourself by stabbing them before they do. This is why we are told not to judge. God knows what people thought, what inputs they received, what state their body was in, and based thereon, what decisions they made - to sin or not to sin. I don't think you can excuse sin on the grounds of a physical malady. What of an alcoholic, then? We don't give them a free run to drink just because they are predisposed. Or if I have a strong drive to lust after women, am I excused of my sin by the body's physiological drives? Of course not.
Besides, Biblically the person is a unity of multiples. The NT has the body, soul and spirit; the OT has the living and dead nephesh with the ruach or breath. This is also why we are told to expect a new body at the Parousia. We are a composite being, a part of which seems to be a material body - so any deficiencies of that body are ultimately part of us, and our responsibility to overcome to become a son of God - as we must overcome our sins eventually through Christ. Afterall, we are told to empty out ourselves, to loose our lives to gain Life. That to me says to overcome our physical limitations, and I see no medical reason to consider them permanent stumbling blocks. They are aspects of our fallen nature, as much as our more psychical drives to sin, like pride or lust or so. The very efficacy of psychotherapy in Psychiatric syndromes shows the power of the will, even in something that seems as intractable as schizophrenia.
We shall stand before our Maker one day, and He will separate the goats from the sheep. He knows which animal was a bit thick in the head, or had a bad leg; but the point was on their decisions to follow the shepherd or not. Perhaps far more virtue was required of that alcoholic to not have that one drink, or that angry man to not lash out, than we realise who do not have those predispositions.
