SAN DIEGO (CNS) - A woman who shot and killed her 74-year-old great-aunt in the victim's Tierrasanta home was in a psychotic delusional state at the time and is not guilty of first-degree murder, her attorney told a jury Tuesday, but a prosecutor told a jury that the killing was premeditated.

"She (the defendant) cannot explain what she doesn't understand," defense attorney Kevin Haughton said of his 26-year-old client, Tiffany Nicole Burney.

Haughton said Burney was in the grips of a psychotic breakdown when she gunned down Daisy Mae Hayes in the early morning hours of Dec. 14, 2011.

"This killing had no rational basis," Haughton told the jury in his opening statement. He said Burney had been hospitalized on-and-off since she started hearing voices at the age of 11.

Burney was diagnosed with paranoid schizophrenia and family members kicked her out of her home because she refused to take her medications, Haughton said. The defendant told Hayes about her problems and the victim prayed for her great-niece, the attorney said.

Haughton said Burney was still in a delusional state when she called police a couple of days after the killing and tried to explain what happened. "She can't explain it because she doesn't understand it," Haughton reiterated.

But Deputy District Attorney Marisa Di Tillio told the jury that Burney planned the murder of her great-aunt, who was the heart of the family. "She (Burney) already had it in her head that she was going to shoot Miss Hayes," the prosecutor said.

Di Tillio said Burney told police she was angry at the victim because she felt she couldn't measure up to her great-aunt, who was known to open her door to all family members.

The prosecutor said Burney showed up at the victim's door sometime after 3 a.m. and asked to use the bathroom.

As Hayes waited on a couch, Burney emerged from the bathroom with a gun drawn and shot her four times, Di Tillio said.

Hayes was at a hospital the previous day taking care of her husband of nearly 55 years, who was recovering from back surgery.

Hayes' body was discovered the night of Dec. 14 in the living room of her Gabacho Drive residence after relatives were unable to reach her and went to the home to check on her welfare.