Olathe ? A man convicted of killing his social worker was sentenced Thursday to life in prison with no possibility of parole for 50 years.

Andrew Ellmaker, 20, of Overland Park, was convicted in May of first-degree murder in the August 2004 stabbing death of Teri Lea Zenner, 26, a Kansas University graduate student. After stabbing her, he used a chain saw to cut her.

He also was convicted of aggravated battery for injuries his mother, Sue Ellmaker, suffered while trying to protect Zenner.

“I’ve been doing this for 15 years now, and I try not to compare cases,” Judge Peter Ruddick said during sentencing, “but I agree, it’s hard to imagine a more heinous, atrocious or cruel crime than what took place here.”

Ruddick could have sentenced Ellmaker to life in prison with no possibility for parole for 25 years. He picked the harsher sentence after hearing an impassioned plea from Zenner’s family for the “Hard 50,” the harshest punishment available in Ellmaker’s case.

“This world will never get to reap the benefits that Teri brought,” Zenner’s husband, Matt, said tearfully. “She’s gone forever and so should the man who murdered her.”

Prosecutors could not seek a harsher sentence because Ellmaker was 17 when he killed Zenner. In Kansas, juveniles are not eligible for the death penalty or life in prison with no chance of parole.

Zenner, who worked for the Johnson County Mental Health Center, was visiting Ellmaker’s house to make sure he was taking his medication.

According to testimony at Ellmaker’s trial, he trapped Zenner in his bedroom. When his mother heard Zenner crying and threatened to call police, Zenner came stumbling out of the bedroom with a stab wound to the neck.

Ellmaker’s mother was stabbed repeatedly after throwing herself between her son and Zenner. After his mother escaped to a neighbor’s house, Ellmaker retrieved a chain saw from his bedroom and used it on Zenner.

Prosecutors argued that the attack was heinous enough to warrant the longer term.

After stabbing her repeatedly, Ellmaker tried to “cut Teri Zenner’s head off with a chain saw, bleeding such wounds that are almost unbearable to look at,” Deputy District Attorney Stephen Maxwell said. “This defendant committed absolute atrocity, a shockingly evil plot.”

They noted in a court filing that answers Ellmaker gave police indicated he was not sure if Zenner was dead before he used the chain saw. At trial, the defense argued that Zenner was dead before Ellmaker used the chain saw on her.

But the defense argued that the lesser term was more appropriate because the murder stemmed from Ellmaker’s mental illness.

“Placing the 25 years is not saying that he will be out in 25 years,” public defender Patrick Lewis said. “It merely gives the parole board the option. It says that at the end of 25 years you will look at him, you will review how he has done within the institution … and determine what is appropriate for the safety of society.”

Ellmaker’s attorneys did not to use their client’s mental illness as a defense at trial, but Lewis noted his client had been committed to a mental institution four times. During a pretrial hearing, the emergency room doctor testified that Ellmaker had been diagnosed with schizotypal, a personality disorder.