Brian,
I'm very sorry for your loss. People who do not suffer from depression find it hard to understand how someone could take their life, understandably. But, it's actually very simple, and I'm speaking as someone with 38+ years with the condition: Human Beings respond instinctively to Pain; they want to get away from it! The example I use with my wife and kids is: Imagine if someone had a blowtorch against your skin; what would your reaction be? Of course they say, "Get away from it as quickly as possible!" Now, imagine if your entire mental state, your LIFE is like that....
When I was 19 years old, I met my first soulmate and fell in love for the first time and we became engaged to be married, much younger than I had planned. Over the next 14 months I watched that lovely little girl turn into a bald skeleton with an enormously swollen abdomen from liver cancer. She's still in the medical books as no one had ever heard of such a thing; didn't smoke, scarcely drank alcohol; how could this even happen, especially having survived an operation that removed 80% of her liver! (The liver regenerates.) I've had pistols against my temple three times; I've had a double-barreled 12 gauge in my mouth once. I definitely understand your sister-in-law's pain and her desperation to get AWAY from the pain. There is only so much the human mind and body can stand.
You're right. Left unchecked and undealt with, depression like alcoholism is often a fatal disease. And, I'm glad that I didn't destroy myself, as God provided me with another soulmate, we've been married over 31 years, have two great kids, and I've even reconciled with God, with whom I was FURIOUS over what Pam experienced. And, my wife knows that I think of Pam every day, with a smile and a sadness, and I look forward to seeing her again. It's toughest to say good-bye to extraordinary people.
P.S. And, I do not collect ANY money from any government or other entity for my affliction. I accept it as the "thorn in my flesh" that HE has given me. So be it. Standing by Pam's side to the bitter end was one of the best things I've ever done in my life. And, I do take medication now and then when it gets really bad. I wish your sister-in-law had, at least the right one for her. Again, I'm very sorry for your loss. It IS brutal on those who love the person, the knowledge of which was the only thing that kept me from pulling those triggers over the years. Remember the opening line of "Sounds of Silence" by Simon and Garfunkel: "Hello Darkness my old friend...."
Kevin