Haku's father and mother were simple farmers, and they lived a peaceful life in the Land of Water.
Background
They loved each other, and were kind to their child. However, Haku's mother was a carrier of a kekkei genkai: Ice Release. She hid this fact from her husband, hoping that the love and peace that was shared in their small family would last forever. One day, Haku discovered the ability to manipulate water. Amazed by this, Haku proudly showed this to his mother, who was horrified by what she saw, and harshly scolded Haku for displaying his ability. Unbeknownst to his mother, Haku's father had seen everything. When Haku's father discovered that his wife and child possessed a kekkei genkai, he assembled a small mob of villagers, and killed his wife. He would have done the same to Haku, but before he could, Haku killed them.
Orphaned, Haku wandered the country looking for a means of survival. The loss of his parents, as well as the indifference with which most people treated him, caused him to seek a reason for existing as well as food and shelter, something he eventually found in Zabuza Momochi.