There are many people who choose to take their own life not because they are suffering from depression. For example, people who are suffering from the early signs of dementia sometimes choose to take their own lives, because they wish to die in a dignified way, and also spare their loved one's having to watch them slowly decay become a husk, a truly terrible thing.
However, there are others, such as depressed people, who wish to take their own lives, but their decision is deemed to be a symptom of an illness. Such people, in choosing to take their own life, do not make that choice with a "clear mind".
People suffering from the early symptoms of dementia are still able to reason, and make rational choices, but depressed people who are suicidal are not making their decisions in a reasoned way. Often depressed people have "exaggerated" and "distorted" opinions and perceptions of the world and themselves, and so their decision making is often based on inaccuracies, which lead them to act in unreasonable ways.
However, you could make the case that a completely reasonable man, who is in his "right mind", has concluded that the world is pointless, existence itself is pointless, and there is no point in continuing, and therefore he wishes to die. In this case its not that his "distorted perceptions" of the world have caused him to take his own life, but rather it was his reasoned conclusion that lead him to make the rational choice to take his own life.
The question, I guess, would be how do we determine what constitutes a "clear mind"?