Authorities say Phyllis Fish was terminally ill and confined to her bed at the home she shared with her husband.
"He wanted to end his wife's suffering because of the terminal illness," said Jim Amormino of the Orange County Sheriff's Department.
Authorities allege Fish gave his wife morphine, then tried to shoot her, but the gun didn't fire. A caregiver in the home then took the gun from him and called 911.
As deputies responded to the retirement community, Fish allegedly used another handgun to shoot his wife in the head, killing her.
"Then he decided that he did not want to live without her and then he turned the gun on himself," said Amormino. "However, based on the law in California, there is no legal excuse."
The medical doctor is hospitalized in critical condition in Mission Viejo.
"As a caregiver, I just think that somebody made a mistake in leaving the gun," said Alice Chandler, a caregiver. "He may have thought he was helping her, but at the same time we don't know what he was really thinking. He just may have been confused."
Fish is now facing a charge of voluntary manslaughter.
"It's a very tragic, sad case all around, but again, based on circumstances and based on suspect's age, it was decided to book him for voluntary manslaughter as opposed to murder," said Amormino.
"I feel sorry for him because if he regrets it, you know, it's too late. He can't bring her back," said Chandler.
At 90 years old, Fish could spend the final years of his life behind bars. If convicted, he could face 21 years in prison.