Edward Thomas Mann, charged with killing three persons and assaulting 23 others last May at the Bethesda offices of IBM, is out of touch with reality and wants to die, either by his own hand or by execution, to prove there is a "global conspiracy" against him, a psychiatrist testified yesterday.

Mann, 38, appeared despondent throughout the first day of hearings in Montgomery County Circuit Court held to establish whether he is mentally competent to stand trial.

At one point, when his sister began to testify, Mann slammed his hand, bound in a cast after the alleged suicide attempt, against the table where he was seated and said, "Your honor, I am going to leave now."

The judge told him he would have to stay, and he calmed down.

Mann's sister, Linda Harris, then testified that her brother had become increasingly isolated after he left IBM in 1979.

Mann has signed a guilty plea and has requested that he be allowed to represent himself in court. Judge William C. Miller, who scheduled hearings after eight court-assigned psychologists and psychiatrists were unable to agree on Mann's mental competency, has said he will not rule on Mann's request until after the hearings are concluded. If Mann is found mentally incompetent, he will not stand trial and will, most likely, be returned indefinitely to Clifton T. Perkins Hospital.

Allen Salerian, a psychiatrist who met with Mann after the suicide attempt, testified that Mann was suffering from a rare form of mental illness. Mann believes that nearly everyone--including his lawyers, wife and the judge--is involved in a conspiracy against him and that the only way to prove this is to die, Salerian said. Mann thinks that if he dies, someone will come forward and confess about the conspiracy, Salerian said.