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.
"As a simple soldier, I learned to carry out orders," Boere testified. "And I knew that if I didn't carry out my orders I would be breaking my oath and would be shot myself."
But the presiding judge said there was no evidence Boere ever even tried to question his orders, and characterized 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."
Boere remained unapologetic to the end for his actions, saying that he had been proud to volunteer for the SS, and that times were different then.
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.
In testimony during his trial, Boere said he remembered his mother waking him up the night in 1940 that Germany invaded the Netherlands and seeing Stuka dive-bombers overhead. Instead of fearing the German bombs, he said his family was elated as the attack unfolded.
"(My mother) said 'they're coming' now things will be better," he told the court, before later adding: "It was better."
After the Germans had overrun his hometown of Maastricht and the rest of the Netherlands, the 18-year-old Boere saw a recruiting poster for the Waffen SS, signed by Heinrich Himmler. It offered German citizenship after two years of service and the possibility of becoming a policeman after that.
He showed up with 100 other Dutchmen at the recruitment office and was one of 15 chosen.
"I was very proud," Boere told the court.
After fighting on the Russian front, Boere ended up back in the Netherlands as part of the "Silbertanne" hit squad.
According to statements Boere made to Dutch authorities after the war, he and a fellow SS man were given a list of names slated for "retaliatory measures."
Boere killed pharmacist Fritz Hubert Ernst Bicknese with a pistol in his pharmacy, then he and the accomplice killed bicycle-shop owner Teun de Groot when he answered the doorbell at his home.
They forced the third victim, Franz Wilhelm Kusters, into their car, drove him to another town, stopped on the pretense of having a flat tire and shot him.
"Kusters fell against the garden door ... and sank to the ground," Boere told investigators. "Blood shot out of Kusters' neck."
After the war, Boere managed to escape the prisoner-of-war camp where he was being held in the Netherlands and eventually return to Germany.
He was sentenced to death in the Netherlands in 1949 later commuted to life imprisonment but the case always seemed to fall through the legal cracks.
The Netherlands sought Boere's extradition, but a German court in 1983 refused on the grounds that he might have German citizenship, and Germany at the time had no provision to extradite its own nationals.
A state court in Aachen ruled in 2007 that Boere could legally serve his Dutch sentence in Germany, but an appeals court in Cologne overturned the ruling, calling the 1949 conviction invalid because Boere was not there to present a defense.
It was after the appeals ruling that a prosecutor in Dortmund quietly reopened the case, beginning from scratch and charging Boere with the three murders in 2008.
During his trial, Boere told the court he was aware of the possibility he would be pursued by authorities, so much so that he never married.
"I always had to consider that my past might catch up with me," he said. "I didn't want to inflict that upon a woman."
