The MD is relevant regarding my knowledge about depression.
I know you can't prove the threshold thing, short of trying to break each and every person on earth. That being said, there are various ways to weaken people (including, for the die hards, prolonged isolation, sleep and sensory deprivation, manipulation, mental and physical torture, destroying their social status, friendships and families... I never practiced this, BTW), and I believe that you can drive anyone to suicide using them.
For example, put someone in a situation where he has the choice between killing himself or killing someone he loves. 5 seconds to decide, after which, if he didn't act, the other person is killed.
This is becoming really gloomy. I balk out.
Edit, actually the bargain I set up above is not a good example of what I meant exactly, but a good example of the level of cruelty you can use.
Edit2: I didn't downvote you, BTW (AFAIK, you can't downvote the answers to your comments).