In Episode 6 ("Unravelling") of Death Note, L reveals that during an 8-day period from Dec 19 to Dec 27, Kira deviated from his usual behaviour by killing 23 "lesser" criminals, and that the reason why he had to kill so many was so that the police wouldn't know which ones were actually used for his plot to kill the FBI agents.
However, we also learn during this episode that nobody other than Light and Naomi knows that Kira has the ability to kill his victims using causes of death other than heart attacks. Since the bus-jacker Light used to learn Raye's identity didn't die of a heart attack, this means Light should have known that the police would not think to link the bus-jacking incident to Kira. If that's the case, then why did he have to kill those 23 other people? As a matter of fact, his doing so allowed L to deduce that an FBI agent met Kira prior to the start of that period. If he hadn't killed those 23 people in that 8-day period, L wouldn't have any reason to suspect this.
So why did Light have to kill the 23? Wouldn't he have been better off carrying out the whole plot with Raye but without that detail?