No, someone cannot avoid torture by committing suicide. Suicide is objectively grave matter and one cannot commit evil in order to avoid being the victim of evil. However, someone who committed suicide because he was afraid of being tortured might not be fully culpable for the sin of suicide. His fear might impede his ability to fully consent to the sin. While the suicide would remain grave matter, if full knowledge and full and free consent were not present, the person would not have committed a mortal sin.
The best and most heroic course of action would be for a person facing the conditions you describe to offer up his fear to the crucified Christ and pray for the grace to endure whatever sufferings come and to offer up those sufferings in union with Christ’s own sufferings.
