My Son, My Son, What Have Ye Done

The film begins with Detective Havenhurst driving with his partner Detective Vargas. They receive a call and drive to the scene of a murder. As they push their way through the crowd at the crime scene, they see Brad McCullum leaving with a coffee cup. Inside the house, the detectives find the body of Mrs. McCullum who has just been stabbed with an antique sword. At the scene are the neighbors and chief witnesses, Mrs. and Miss Roberts. The detectives soon realize that they had just seen the murderer leaving the scene.
The Robertses tell the detectives that Brad was disturbed, and had changed when he went to Peru recently. In a flashback we see Brad in Peru preparing for a kayak trip on a raging river. Back to the present time, the police have learned that Brad has taken two hostages in the house across the street. The police surround the house, and Brad's fiancée Ingrid arrives. Ingrid talks to Havenhurst about Brad's trip to Peru, saying that Brad's friends all drowned on their kayak trip, which Brad had decided at the last minute not to take part in; he later claimed that the voice of God had told him to stay behind. Several more flashbacks follow of Brad and Ingrid in Brad's bedroom, talking with Mrs. McCullum, looking at nearby houses, having dinner. Back in the present, Brad demands pizza for himself and the hostages, along with a car for transportation to Mexico.
In another flashback we see Brad in rehearsals for a Greek tragedy directed by Lee Meyers. As the pizza is delivered to Brad, Lee arrives at the scene of the crime. Lee talks with Havenhurst about Brad, and we flash back to Lee and Brad visiting Uncle Ted's ostrich farm. Brad convinces Uncle Ted to give him the antique sword which would be used in the crime. Brad uses the sword in more rehearsals for the play, in which he plays the part of a man who kills his mother, who is played by his fiancée Ingrid. Brad becomes disruptive and is eventually kicked out of the production, but still travels to Calgary with Lee and his mother to attend a performance. We see some footage of Brad at Machu Picchu, and then at a Central Asian market.
A SWAT team arrives to take command of the hostage situation, and the detective talks further with Ingrid and Lee. We see a flashback to Brad and Ingrid's trip to Tijuana, after which they go to Bob Wilson Naval Hospital to "visit the sick in general". Brad buys several pillows at the hospital gift shop. Then Brad and Ingrid walk in Balboa Park, and Brad gives away his bag of pillows, keeping one, and leaves his basketball in a tree.
Back at the crime scene, Havenhurst interviews Miss Roberts, who had witnessed the crime. In a flashback to the scene just before the murder, we see the Robertses sitting down with Brad and his mother for coffee. When Brad steps out, his mother tells Mrs. Roberts that Brad has just tried to smother her with a pillow. Brad gets his coffee cup, and then goes to his car and returns with a baseball bat and the sword. He hands the bat to Miss Roberts, saying "kill me before I do it". She does nothing, and he draws the sword and holds it in front of his mother. Miss Roberts tells detective Haverhurst that Brad stabbed her, though we do not see the crime on camera.
Ingrid and Lee talk to Brad, urging him to release the hostages and surrender. Ingrid realizes that Brad's hostages are his two pet flamingoes, and the SWAT team moves in and arrests Brad. As Brad is led into the car, we see shots of running ostriches. The final shot is in Balboa Park, where a young boy resembling Brad picks up the basketball.Source: Wikipedia

Question: What was the presumed cause of death?