Haku is a child who was raised to believe that "bloodline limits" (...forget the translation...so...it's literal for
now...) are dangerous, not to be trusted and not something anyone should want. When Zabuza decided to take Haku in--
and wanted him for his bloodline limit-- Haku had to reevaluate his oppinion. The child concluded that people need
to be accepted (needed). "When a person has something important to protect, ... that is when they can truly become strong."
So Haku fights for Zabuza because he cares for him.
He became an amazing fighter at a very young age for his emotions.
"I want to protect someone precious to me. To work for that person. To fight for that person. To make that person's dreams
come true. That is my dream."
Itachi, on the other hand. . .
He doesn't seem to show any kind of emotion, and his motivation tends to be along the lines of, "I wanted to test my
strength." If he stayed with the Uchiha family, he reasons, he would not have gotten any stronger.
So, Itachi rids himself of his "precious people" first by killing Shisui-- to get stronger, of course-- and then his
family. But, interestingly enough, he leaves Sasuke alive.
We can only speculate why he did that, of course.
The question, to me anyways, then becomes:
Who's the stronger one?
The dead child-- who gave his life in pursuit of his dream-- or the mass-murderer-- the one who's forsaken everything
for a legend?