A frail, repentant Roswell Gilbert regained his freedom Thursday after spending more than 5 years in prison for the 1985 ``mercy killing'' of his ailing wife. The 81-year-old Gilbert acknowledged publicly for the first time that he was wrong to kill his wife of 51 years, who suffered from Alzheimer's disease and osteoporosis. ``I shouldn't have killed my wife, now I know that,'' said Gilbert, whose case focused national attention on the killing of an ailing loved one. ``It's difficult to explain to anyone who hasn't been through this trauma, a mental trauma really,'' said Gilbert. He said his wife's illness ``created just a complete state of desperation in my mind ... it's a lousy excuse, but that's what it was.'' Gilbert shot his wife, Emily, twice with a 9mm Luger in March 1985 in their Fort Lauderdale apartment. He was serving a mandatory minimum prison term of 25 years on a murder conviction when he was granted clemency on humanitarian grounds Wednesday by Gov. Bob Martinez and the Florida Cabinet. Gilbert, who suffers from heart and lung disease, wants to regain some of the 40 pounds he lost in prison and ``recoup my health.'' He chuckled at reports that his death may be imminent. ``I'm a tough old rooster ... don't believe it,'' said Gilbert, who said he would probably ignore his daughter's pleas to stop smoking. ``Daddy has very bad emphysema and one of the main things I'm going to do is really insist he quit smoking,'' said daughter Martha ``Skipper'' Moran, the Gilberts' only child. The retired engineer was examined in January and a medical report showed he suffered from heart and lung disease and was considered ``at high risk of death at any time,'' because of his age and physical condition. Gilbert, who left the North Florida Reception Center with a handful of possessions in a small brown paper bag, described prison as a ``human zoo.'' ``It's been plain awful for a freedom-loving person like myself,'' he said. Besides a hamburger, a shot of bourbon and a nice soft bed, Gilbert said he was looking forward to seeing his three grown grandchildren soon. ``I haven't had a good look at them for a long time,'' he said. He left the prison for a friend's home in an undisclosed location. His daughter said she expected him to resettle in Fort Lauderdale. The lawyer who prosecuted Gilbert at the murder trial, Kelly Hancock, said he was relieved that the state decided to free Gilbert. ``His health is deteriorated, and I think his release puts to an end a very, very tragic case _ a case that made us all more aware of the problems of the sick and the elderly have,'' said Hancock, who's now in private practice and publicly sought Gilbert's release in recent weeks. Actor Robert Young, who portrayed Gilbert in a 1987 television movie, ``Mercy or Murder?,'' also said he was elated with the news of Gilbert's clemency. ``That's sensational,'' said the 83-year-old Young, best known for his roles in the TV series ``Father Knows Best,'' and ``Marcus Welby, M.D.'' Although the two men never met, Young said he spoke with Gilbert many times by telephone. ``We had wonderful chats,'' said Young. ``There was no self-pity in him at all. It was a remarkable experience for me to play the role and get to know the man.'' Mrs. Moran, who lives in Baltimore, said she believes her mother would say: ``It's about time they let him out.''