Cancers constellation is made through a story of fighting. Cancer which is represented as a giant crab in the myth was sent to attack Hercules during his second out of twelve labors. He was sent because hercules killed his family. Hera sent the giant crab to fight him back but he died when hercules took club to the crab. This myth does sound very harsh but that was how the constellation came to be.
Even though most people believe the myth of how Hera sent the giant crab to attack Hercules as the way the cancer constellation was created there are other beliefs. I believe in this myth of how the constellation was created:
The twins(known as gemini) were walking along the beaches of the world
with Leo the Lion by their side. When all of a sudden they heard a scream
coming from behind a mountain. they zoomed off in the direction it came