If you decide to leave a note the note should answer every conceivable "If I had been nicer to them, would that have made a difference? If I had did that favor they asked for that one day, would that have made a difference? Was there something I could've done to prevent this?" type of question.
I don't know what is motivating your decision but if I were to leave a note I'd make it clear that it wasn't people, or this or that. I'd make it clear that my depression got so bad that my mind is no longer working the way it used to and this fact has literally made life unbearable, I'd explain that this is a serious, legitimate illness, no, pills and medication would not have made a difference, I'd make it crystal clear that this was my decision, that it was a well thought out decision, I'd ask them to appreciate each other and offer each other support during this time, I'd ask them to take comfort in the fact that my suffering due to my illness has ended and ask them that instead of grieving or torturing themselves over what they could've done to prevent this (nothing) to instead honor my memory by living their best life possible and to embrace love, joy, good times, laughter and to appreciate each other.