You really don't know what it's like unless you've been there.
This* is the most accurate description of what it's like to live with clinical depression I've seen--at least, based on my own experience with it. *(
Trigger warning: descriptions of self harm.) But it's still hard to really grasp unless you've experienced it.
I used to think the dark things came from demons encouraging the dark thoughts, but...IDK, that seems like saying "the devil broke my leg", or "the devil caused my heart attack". Bodily injuries and illnesses happen, mental illnesses happen, and it's not always about demons. Not to say they can't influence things, but eh.
Correct, they don't. You don't start out just wanting to die. The depression wears you down, all of your reserves of strength, until you begin thinking about dying, considering the pros and cons. As time goes on, you begin to long for it. Eventually, you're pushed so far you will go through with it.
Of course there are more issues than just depression, but that's the one I'm most familiar with...and I'm sure there are similarities, anyway. I would think you would have to be in great mental anguish to die, if that weren't so, there'd be no reason to want to escape so badly.
Also, suicidal and mental issues doesn't always mean committing crimes...though there are individuals there, too. Not that I'm an expert, because I'm not, but I would say that functional issues are also at play there--simply because no one in their right mind is going to kill themselves.
I'm not saying that the Word of God has no power over this, no. That'd be like saying God can't heal a cancer patient...of course He can. But I'm not for telling individuals in this state of mind that they're there because they don't have enough faith. An individual isn't any more at fault for having mental illness than they would be for having cancer.