Why he didn't kill her when she slapped him? It was because he thought he could need her one day, or maybe even though he is a shaman whose only purpose is to dominate the world (his world), he has been born as a human being and human beings simply enjoy having fun once in a while. Plus Anna was the only girl around, strong and bold enough for him to look at her. Hao loves to dominate. He knows everything about everyone. He knew that Anna is a wise, strong , determined women and everyone fears her. He showed her he's not like the others, when he blocked her firs slap. He has always had a calm personality and the sarcasm you hear in his voice is driving people insane. So in other words he wasn't suggesting her to marry him( not seriously anyway) nor did he try to get her to think he is better than Yoh, he was just playing with her. He is a man after all. Most probably he found her attractive and told himself:" why not?" He also liked her confused face when leaned in still holding her wrist. Anna meant nothing to him. Later he tried to kill her, but let's be serious. Is there any person in this world Hao cares for? Anna was nothing but a bonus in his game. He had never seen a potential girlfriend (or how would he call it) in her. Hao always get anything he wants. If he wanted Anna he would have showed up like he had done with Ren for example. She was funny to play with, everyone else feared him, Anna on the other side was only annoyed by him. And yes, Hao is the most powerful shaman that ever lived, he has a great number of followers but in first place he is a men. Anna , despite of the fact she would never admit it, she enjoyed his attention because she is a girl, a teen. He is strong, feared, dangerous and.. good looking. (If she likes Yohs appearance , she can't say she doesn't like Hao's.) And Hao even though never mentioned ,seems to take care of his body. He has got a six pack, great hair , you have to take car of such a long hair. But the simple fact that he think about himself as being perfect makes people believe he wouldn't forget about his human side(his body). Anna also seems to be pretty embarrassed and shy when he leans in only few inches away from her face and the shaman surely notice that. And he liked it.

So the answer to the question: Why he didn't kill her when she slapped him, it's because he didn't take her seriously one second . He knew he got her, at least confused and that was enough for him. Also his message for her wasn't a very important one because she had been on her way to Yoh with that book anyway. But he thought it could be funny to meet her. During the whole conversation he had his usual smile on his face. He noticed that Anna lost for few seconds, before hitting him. He knew he got her where he wanted to. He was aware she isn't going to respond him the same way. She wasn't going to flirt back , but that was fine with him. And the smile that formed on his lips after getting hit by the girl was a kind of "I was expecting that".

Hao wasn't hoping for a "shaman like" conversation. He only told her things she already knew about (most of them). When he first met Anna he showed off 2 of his human sides. Flirting and of course curiosity. He was curious about her, he wanted to meet her in person and as I said have a little" boy like" fun.

What do you think? If Anna didn't slapped him, would he have kissed her?