At some point we come crashing into the Philosophy classroom to discuss free will and individual versus societal and other environmental causes, and individual character-nature versus individual neuron-nature and so on.
You are who you are. You either are or are not responsible for your behavior, but we have a lack of consensus on that before we even get to the specific question of “mental illness”, don’t we?
Generally, legally, we hold that if you are adult, you are responsible for your behavior unless you are not, a less than useful construct I suppose, but revolving around questions of capacity. We have capacity standards for whether I would be let off the hook for shooting holes in people and eating them for dinner while cackling should be used in determining whether or not psychiatric meds should be administered to me over my objection, and whether or not I can make a valid will or sign a contract to buy a car down at the Chevy dealership.
Unfortunately, they don’t tend to be the same standards, and the standards for the involuntary psych meds stuff, in practice, totally sucks.
Since I’m not going to argue that everyone should be electrocuted as murderers if they kill someone, or that contracts should be binding on anyone who signs one, it would not make sense for me to argue that no one could ever lack capacity to make decisions with regards to psychiatric meds. But I’d formalize the process like this:
a) Does the individual know what the medication is, and what it is prescribed for? They need not be capable of drawing the carbon chains on a blackboard, nor need they cite the DSM code or recite the full PDR entry on the pharmaceutical in question, but they should be able to say, in effect, “The doctor believes that I am sick in the head and the medicine will make my head work better”.
b) Can the individual express a rational reason for deciding not to take the prescribed medication? The reason need not match the judgment that the doctor, the judge, or even the hypothetical “reasonable person” on the street would give, but it needs to make sense. “I don’t want to take the pills because they make my fingernails pregnant and they bite and you did this to me” would not cut it, but “I do okay without the pills and I don’t like how they make me feel” is certainly sufficient.
And, having made that decision, the individual is responsible for what they do and should be held accountable for it – but, again, according to the same rules as anyone else.