A successful insanity plea doesn't mean that the court is saying the person is not responsible for their actions; it just means that mitigating factors (in this case, something that sounds a lot like mania and/or psychosis induced by antidepressants, which is a well-documented phenomenon) made the defendant unable to control his actions and/or understand the illegality of them. (I'm not sure what CT's definition of legal insanity is, but most states use the M'Naghten Test, the irresistible impulse test, or some combination of the two in their statutes.)

This is different than using illegal drugs, too. One presumes that in the case in question, the medications were legally prescribed by the defendant's doctor, who judged that the risks outweighed the potential benefits. When a doctor prescribes a medication, most patients assume that means it's reasonably safe to take said medication. This is not the case with illegal drug use: those drugs are not shown to be safe, nor are they legally prescribed.

Also, bear in mind that a successful affirmative defense does not mean there are no consequences for the criminal behavior. A successful plea of not guilty by reason of mental disease or defect means the defendant will be confined indefinitely to a psychiatric unit or hospital. Often, they end up confined for much longer than they would've if they'd been sentenced to prison for the crimes they committed, and they have fewer legal rights and protections than convicted felons. Contrary to what you see on TV, and insanity defense does not get you off free.