Before Adam sinned, he was a completely spiritual, rather than a physical, being. His garment consisted of light, and he encompassed all worlds. After he sinned, his size was diminished, and his body became physical. The soul of Adam included many souls that were to come after him. In blemishing his soul, he blemished all of these souls.
After Adam separated from Eve for 130 years, he was visited by she-demons. The generation of the Flood consisted of souls that were created by Adam from his unions with the she-demons. These souls needed to correct the blemish of the way they came to earth, and they all came back at the time of the flood. However, instead of correcting themselves, they ruined themselves even further, and thus the Flood was needed to erase their bodies and the Earth three hand-breadths deep.
While Adam was commanded to be a vegetarian, Noach and subsequent generations after him were allowed to eat meat. Nevertheless, since every living creature was created for a purpose, it is forbidden to kill it without a need.
The Arizal himself was careful not to kill any living creature, however small or primitive, such as mosquitoes, fleas, and flies, even if they caused him pain. In fact, those flees that annoy a man are for his correction and benefit, and one will do good not to kill them.