HOUSTON — Andrea Yates' psychotic mind was like her own private battlefield in the war between good and evil, according to a defense expert who testified Thursday that Yates believed killing her five children would be a final defeating blow to Satan.

"It is my opinion, with medical certainty, that Mrs. Yates on June 20, 2001, believed she was doing what was right for the children. And in her heart, and even in her psychotic mind, she knew she was doing what was right," said Dr. Phillip Resnick, a forensic psychiatrist who evaluated Yates in 2001.

Yates is accused of drowning her five children, Noah, 7, John, 5, Paul, 3, Luke, 2, and Mary, 6 months. She has pleaded not guilty by reason of insanity.

Prosecution experts say that even if Yates were mentally ill, she knew that killing her children was wrong, and therefore, she does not meet Texas's legal standard of insanity.

"In my opinion, she did know what she did was illegal, even though she thought it was right," Resnick said.

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Yates even expected to be executed, Resnick said, and her death would fulfill her delusional prophecy, as she believed Satan already had won the battle for her soul.

"Satan, the one and only Satan, not just a demon, but the singular Satan, would be executed ... It would fulfill the prophecy, the final battle, Armageddon," Resnick said.

He added that those who know their actions are wrong tend to wipe away fingerprints, wear disguises, and try not to get caught. But Yates hid nothing: She called 911 after the killings, and she cooperated with investigators, Resnick said.

Based on interviews and his study of the defendant's psychiatric records, Resnick said Yates believed killing her children was the only way to win her battle with Satan for her children's souls — if she killed them while they were still under the "age of accountability," they would join God in Heaven.

Yates had a history of postpartum psychosis, major depression and two suicide attempts before she drowned her children and laid their bodies on her bed, covering them with a sheet. Noah, the eldest, was left floating face-down in the tub.

Before the killings, Resnick said, Yates saw messages in the movies "Seven" and "O Brother, Where Art Thou?" that she believed were Satan's personal torments.

"Innocent events — anyone could go to a movie, but we wouldn't get the message that this was for us," Resnick said.