I didn't understand why kaname changed so much and try to kill all purebloods....
He even makes yuuki feel sad and that's so not like him, so why?!

________________
He stepped back and threw his arms out.
"I'm always crazy around you Rose. Here, I'm going to write an impromptu poem for you."
He tipped his head back and shouted to the sky:
"Rose is in red
But never in blue
Sharp as a thorn
Fights like one too.” - Adrian; Shadow Kiss


http://www.mangareader.net/782-43930-9/heart-no-kuni-no-alice/ chapter-16.html

To keep you wondering what comes next lol
Anways its got me wondering 2

I made a long post in the discussion on chapter 71 that pretty much answers your question here:

http://www.mangaupdates.com/showtopic.php?tid=26233

There's still a lot that we don't know about regarding Kaname's motives, but just because we don't know for sure why he wants to do this, it doesn't mean he's suddenly OOC. In fact, I wouldn't say Kaname has changed at all. As I said in my post on 71, Kaname has been talking about this plan--killing the Purebloods--for a long time now. It wasn't until the past 5 chapters or so that we finally found out "killing Purebloods" was the plan that he kept hesitating on.

A lot of people assumed Kaname's mysterious plan was "protect Yuuki" and left it at that, but you have to figure that if Kaname was hesitating to put it into action, it was because he knew it would bother Yuuki. If his plan was only "protect Yuuki" that wouldn't have bothered her, right?