Confused about Rem's actions in episode 25

!I haven't seen past 25, so maybe this will be revealed later. But why did Rem kill Watari and L to save Misa from the results of L's execution test when it would have been smarter to manipulate the test results by killing the death row inmate on the 13th day after they used it for an execution? !<

!Because now it seems like the former police officers investigating can follow L's lead and further investigate the 13 day rule and convict Misa.!<