DOVER, Del. » A former soldier who said he became “addicted to killing” while serving in Iraq and Afghanistan pleaded guilty but mentally ill today to charges of raping and killing a Delaware woman.

Dwight L. Smith Jr., 28, entered the plea at a Superior Court hearing in Wilmington. He faces a mandatory sentence of life without parole when he is sentenced April 22.

Smith was charged with killing Marsha Lee of Wilmington while he was on leave from Fort Drum, New York, in December 2011.

Lee, 65, was abducted while walking her dog. Police said at the time that Smith had targeted Lee at random after deciding he wanted to kill somebody. Authorities said he hit Lee with his sport utility vehicle, raped her and beat her to death.

According to prosecutors, Smith said in the confession that led to his arrest that the attack was unplanned and random.

Prosecutors were seeking the death penalty, but deputy attorney general Steve Wood said today that they decided to offer a plea to a life sentence at the request of Lee’s family.

“They are aware that criminal cases can drag on for a decade or more and they want closure,” Wood said in a text message to The Associated Press.

After Lee’s death, Smith wrote to his father about his mental health and suggested that he became “addicted to killing” while in Iraq and Afghanistan.

Smith’s attorneys have said he has serious mental health issues.

In the letter to his father, Smith complained that his mental health was deteriorating and that solitary confinement after his arrest was “starting to play tricks with my mind.”

“I just don’t want to be locked up without getting some type of help,” he wrote.

In the letter, Smith also wrote of killing a lot of men and children, some after they begged for mercy.

“I think I got addicted to killing people. … It got normal for me to be that way. I never wanted to be this way. I just took my job way to (sic) serious. I took things to the extreme. Anyone can tell you that I changed. It is like being a completely different person.”