The Kree were known to be spawned over 6 million years ago, along with the Skrulls and all other forms of life, including humans. The makers of the Kree and all other life were the Xorrians, who supposedly created them out of their own DNA.
Historically, the Kree were experimental subjects to the Celestials, who are known as the all powerful “space gods”. From then, the Kree mainly lived on the prehistoric planet of Hala, which was home to the Kree and and the Cotati. At this same time, the highly intelligent Skulls race, which are a race of humanoid and space faring reptiles, came across the Hala. The Skulls were mainly focused on peace at this time and wanted to establish education in order to successfully trade with Hala.
To prove which race was worthier to conduct business with, the Skulls put the Cotati and Kree on oxygen filled “blue area of the moon”. The Skulls decreed that whichever race created something grander with their resources would be deemed the worthier race. The Kree created a prosperous city, while the Cotati made a intricate, beautiful garden.
When the Skulls returned to the Blue Area of the Moon one year later, they decided the Cotati was the more superior and worthier race; this greatly upset the Kree and they attacked the Skulls and killed the Cotati and stole the Skull’s ship. The Kree returned to their native planet of Hala and spent time deciphering the technology of the star-ship. Within a hundred years of acquiring interstellar technology from the Skrulls, the Kree empire began to spread throughout the Greater Magellanic Cloud.
From this they launched an attack on the entire Skull empire and have been at war with them ever since. In addition, the Kree were also the creators of the Inhumans – the Kree used prehistoric humans from earth and put them through the Kree experiment, making them Inhumans and ultimately enslaving them to be used against the Skulls to fight. The Kree wanted to use the Inhumans as soldiers but shortly after the created them, they claimed that the Inhumans experiment was a failure and they ceased all production. The Inhumans returned to earth after this and the to blue area of the moon where they live now. Overall, the Kree are a complex alien race that still lurks throughout the galaxy. Hopefully more is discovered about this mysterious race!