What is it that makes Lennie kill Curley's wife?

Lennie kills Curley's wife in a panic.... it isn't intentional, he just wants to stiffle her screams. As we've learned throughout the story, Lennie doesn't know his own strength.

As far as you second question, Lennie knows exactly what to do because George reminds him of it continuously..... Lennie was to go back to the river and hide. George would meet him there.