CAN A DEMON KILL SOMEONE OR MAKE HIM COMMIT SUICIDE?

Once again, a demon is not given this power unless God grants it for some reason, but it is hard to imagine why God would do that.  The highly unusual case of the exorcism of 23-year old Anneliese Michel in Klingenburg, Germany, in 1976 may be taken under this rubric; she died of starvation in a possessed state after a year of exorcisms, but I doubt that the presence of the devil victimizing her is to be considered the actual cause of her death.  there were various medical and spiritual factors that led to her demise, all of which are dealt with in the excellent book, The Exorcism of Anneliese Michel, by Felicitas Goodman.

Fr. Gabriele Amorth of Rome says that it is highly unlikely that a person would commit suicide once he enters into the care of the Church.  This spiritual agreement between a possessed person and the Church exerts a restraining force on the demons even before the exorcism formally takes place.  The devil can certainly tempt people to suicide, however; he cannot make someone commit suicide Jesus does not call him "a murderer from the beginning" without reason, and certainly suicide is one of his tools; however, neither must we allow ourselves to believe that the devil has more power than he actually has.

Fr. Thomas J. Euteneuer, "Exorcism and the Church Militant" page 37.