Due to an odd hearing defect, the man had misinterpretted everything he had learned in his Flower Arranging class. You see, his eardrums were a little too long, so he thought the word 'flower' was the word 'bone'. You can imagine how embarrassing that was in his youth.
When the man graduated from his college course on Flower Arrangement, or Bone Arrangement as he called it, he promptly went out and killed some people and some animals. Because of his inadequate eardrums, he thought 'Buy flowers in a flower shop' sounded like 'Go out and kill some people and animals and take their bones.'
The man then returned to his apartment and stripped the flesh from all his victims, and removed their bones. He placed these in a vase, and arranged them quite quaintly. Later, the police arrived and took him to jail, and then to court. When the judge asked him, "Why did you kill a bunch of people and animals and then arrange their bones?", the man replied, "Um... thirteen?"