describe lennies killing of curleys wife and why the author focus the readers attention to lennie instead of the dieing women

For a description of the murder, you should read gradesaver's chapter 5 summary and analysis;

http://www.gradesaver.com/of-mice-and-men/study-guide/section5/

As for the focus being on Lennie rather than Curly's wife, that's simple. This isn't about Curly's wife; she's a prop in the story. This scene is about understanding the actions of someone who truly doesn't know what the result of his actions will be. It's all the more a tragedy because Lennie doesn't mean to kill anyone..... he never has.