TRENTON, N.J. (CBSNewYork/AP) – At sentencing hearings in 2006, serial killer nurse Charles Cullen did not explain why he killed at least 29 hospital and nursing home patients in New Jersey and Pennsylvania.

He had told investigators they were mercy killings. But a prosecutor said Cullen was driven by a “compulsion” to kill and was no “angel of death.”

In an interview to be broadcast Sunday on CBS’ “60 Minutes,” Cullen at first says he thought he was helping people by ending their suffering. Many of the victims of his lethal drug overdoses were old or gravely ill.

But Cullen tells a different story when reminded some victims were not close to death or in great pain. He says there was “no justification” and “I felt overwhelmed at the time.”

The interview airs this Sunday at 7 p.m. on CBS.

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Cullen is serving multiple life terms at the New Jersey State Prison in Trenton.(TM and © Copyright 2013 CBS Radio Inc. and its relevant subsidiaries. CBS RADIO and EYE Logo TM and Copyright 2013 CBS Broadcasting Inc. Used under license. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.)

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