What was the reason for Harada's final decision?

Question

In The Wolverine, Kenuichio Harada is the head of a band of ninjas that has defended the Yashida family for 700 years. In the early part of the film, he uses his skills in a failed effort to protect Mariko Yashida from kidnapping by yakuza. Later, he uses his skills to capture Wolverine, who was wanted by Mariko's grandfather so that he could steal the mutant's healing factor.

In the climactic battle, Harada suddenly switches sides, attacking the elder Yashida in defense of Wolverine. Why? How are the interests of either member of the Yashida clan advanced by this?

Answer 1

As was shown in the film, Harada was Mariko's lover at one time, and still loved her. She wanted to save Wolverine, and this prompted Harada to side with Mariko, and attacks Yashida so that Mariko can save Logan.

Answer 2

In addition to kicker86's answer Harada probably also realized that Yashida has finally turned mad out of his fear of death.

He maybe had to fight an internal struggle all along. While Yashida was his master and the Black Ninjas always served his family, he also realized that his recent ways were a bit skewed. This is evident in his dialog with Mariko when he says to her:

This isn't easy for me, but I'm following Grandfather's wishes. The Viper woman is evil, we both know that. But she's a means to an end...an end your grandfather wanted.

So while he still believed in the cause, he already doubted the measures. But once he saw how Yashida turned into that giant monster and didn't care about anything apart from Logan he saw that this isn't the way to go. And when attacking him he directly said to Yashida:

This is not the way!

Yet kicker86's explanation might still be the main reason why he finally decided to turn against his master, even if not the only reason.