While awaiting his trial, Nilsen decided to dispense with his legal aid, Ronald Moss, but then reinstated him. Nearing the trial date, he fired him and hired Ralph Haeems, the lawyer of a prisoner with whom he was in love, David Martin. Haeems decided to go for a "diminished responsibility" defense, citing a mental abnormality in Nilsen. His defense counsel was Ivan Lawrence, asking for a charge of manslaughter.
Nilsen examined the crime scene photos and felt ill over his atrocious acts against others. He wondered if the victims' families could ever forgive him.
He wrote over fifty notebooks of his memories to assist the prosecution, and also drew a series of "Sad Sketches" showing what he had done to some of his victims.
On the eve of his trial, he wrote, "I have judged myself more harshly than any court ever could."
Nilsen was charged with six counts of murder and two charges of attempted murder. Alan Green was the prosecutor. He maintained that Nilsen had killed in full awareness of what he was doing and should be found guilty of murder. His principal evidence was from Nilsen's lengthy statement to the police, while the defense relied on psychiatric analysis.