Light tricks Rem into killing L. Light makes Rem think she has no other choice but to kill l in order to save Misa's life. This happens when l tries to test the death note to see how exactly the death note works, and then Rem kills l before he can test it.
DANG REM!!!!!!! You know, as much as I like Misa, I think it would have been better for Rem to kill HER instead!! That would have saved EVERYONE SOOO much mais trouble!!!! I mean, it would be all unexpected and tragic and stuff, but at least l and Watari would be alive!!
stupid fricking Rem did!!! Dx she also killed L!! D': although, Light killed them both indirectly, because he made it so that Rem would have to in order to "save Misa". xP