Edward Thomas Mann, accused of killing three persons during a rampage at the Bethesda offices of IBM, has said he would become a "killer" and kill again if he is not allowed to represent himself and if a signed guilty plea is not accepted, a psychologist testified yesterday.

Ellen McDaniel, a professor of psychology at the University of Maryland Medical School who examined Mann, quoted Mann's alleged threat in the third day of hearings before Judge William C. Miller in Montgomery County Circuit Court. The hearings are being held to determine whether Mann is mentally competent to stand trial.

Mann, 38, is suffering from paranoid schizophrenia and believes that he can expose a conspiracy that he is convinced is being directed against him only if he is allowed to plead guilty and die, McDaniel said.

McDaniel was the fourth psychiatric defense witness to testify that Mann's belief--that a global conspiracy is being directed against him--is evidence that he is incompetent and incapable of assisting in his defense. That alleged conspiracy, McDaniel said, extends to the court and Mann's lawyers--who are presenting a defense he repeatedly has stated throughout the hearings that he does not want. In these hearings, Mann views the prosecutors as his allies and the conductors of his wishes, McDaniel said.

McDaniel also testified that Mann viewed the shootings at the IBM building last May as the conclusion of more than two years of an "honest effort to play stupid with the system" so he could sit back and catch the alleged conspirators. He told her, however, that he could not remember shooting anyone on May 28, the day of the incident, McDaniel testified.

Under cross-examination, prosecutor Michael Mason attempted to discredit McDaniel's testimony. He noted that her diagnosis differed from that of at least 10 other psychiatric witnesses who had found that Mann was suffering only paranoia.

David Rojas, a psychiatrist at Clifton T. Perkins in Jessup, Md., where Mann is being held after an alleged suicide attempt at the Montgomery County Detention Center, testified that, on the basis of two conferences with Perkins staff members, he believed Mann was coherent and capable of assisting in his defense.