Hi,
Satan cannot enter the soul unless one invites him. To take one’s life is a mortal sin in itself because only God, who creates human life, has the right to take it. To take one’s own life is to go against the very strong instinct that we have to preserve our life. So those people who do, usually do so to stop some extreme pain that can be psychological or physical.
They are therefore, not intending to offend God, but to relieve pain.
Fr. Vincent Serpa, O.P