Boere died yesterday of natural causes in the facility in Froendenberg where he was being treated for dementia, North Rhine-Westphalia Justice Ministry spokesman Detlef Feige said. He had been the state's oldest prisoner.
Boere was on the Simon Wiesenthal Centre's list of most-wanted Nazi war criminals until his arrest in Germany and conviction in 2010 on three counts of murder.
During his six-month trial in Aachen, Boere admitted killing three civilians as a member of the "Silbertanne," or "Silver Fir," hit squad a unit of largely Dutch SS volunteers responsible for reprisal killings of countrymen who were considered anti-Nazi.
He sat through the proceedings in a wheelchair and was regularly monitored by a doctor. He spoke little, but told the court in a written statement he had no choice but to obey orders to carry out the killings.
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But the presiding judge said there was no evidence Boere ever even tried to question his orders, and characterised the murders as hit-style slayings, with Boere and his accomplices dressed in civilian clothes and surprising their victims at their homes or places of work late at night or early in the morning.
"These were murders that could hardly be outdone in terms of baseness and cowardice beyond the respectability of any soldier," the judge said in his ruling. "The victims had no real chance."
Born to a Dutch father and German mother in Eschweiler, Germany on the outskirts of Aachen Boere moved to the Netherlands when he was an infant.