What might cause a devotee to commit suicide and what are the consequences? Can he do that in the name of Krishna?

One may think that suicide will bring relief from the sufferings we're undergoing, But that's not the case. While the gross body dies, the subtle body (the mind, the intelligence, and the false ego) carries our thoughts and desires onward. If one commits suicide, he interrupts his own spiritual progress. We are not meant to take our life in our own hands, disrupting the karmic cycle.

Scriptures say that those who commit suicide will have to suffer in a ghost body in their next lifetime; this means life without a material body, but with material desires. It's a hellish existence and can go on for a very long time. In such a condition, one sometimes desires to take another person’s body in order to dull the pain. Please remember that the Supreme Lord, as the Supersoul, is always with us as a witness. He knows, better than anyone, our unique good qualities as well as the extent of our pain. He is constantly arranging circumstances to purify us. He goes with us from lifetime to lifetime.

Not knowing the plan of the Lord, we shouldn't intervene. Under no circumstances is suicide permitted. We are Krishna’s and this body is His. Although material life can be very painful sometimes, if we persevere and take full shelter of the Lord, we will find there is light at the end of the tunnel. If you are actually contemplating such a thing, please seek professional help.