[Contains spoilers!] L and Valentina have always been good friends, no doubt about that. Either of them would go to extraordinary lengths to protect the other, so when L is given the Kira case, Valentina can't help but go snooping. It wasn't her intention, however, to discover Kira in the process. But when Light finds out Valentina knows his secret, he must go to great lengths to stop her. If that means killing her, fine by him.