In other words, the more determined and less flexible the person's context, the less the capacity for "independent" decision-making or choice-- in any sense of the term we might be able to agree upon.
Of course, not all depressives suffer from the same kind or degree of depression, so I'm not saying that choice doesn't enter into things in all instances. And I'm not saying that anyone on the board has diminished capacity, in either a legal or ethical sense. Depends upon the nature and severity of the illness.
I'm only mentioning this to suggest that depression covers a wide range of illnesses, some of which imply a broader range of choice than others. (Within a given class of mental illness, there's often enough deviation within the class to make a precise diagnosis difficult, and sometimes symptoms of one class lead to confusion with another class; I've known people who suffered from a specific sort or another of depression but were misdiagnosed as bipolar, and sometimes a full-on psychotic gets misdiagnosed as a psychotic depressive, and vice versa, and so on.)
Ya'll can go back to bickering. Just wanted to throw these things out there.

EDIT: Assuming that a person tries to or succeeds in doing something bad to himself, herself, or others, I wouldn't say that the someone in question can or should use depression or anything else "as an excuse" for bad behavior. That's more of a "social good" issue, if you follow me. Whether or not someone has diminished capacity has little to do with coming up with a way of dealing with what the person did, although it might (or might not, depending on your social views) have to do with the manner in which society deals with the person.