In the first scan she may have been saying she wanted to kill them to scare them,but notice in the second scan she isn't talking to Naruto and Sasuke,she's talking to Zetsu.
She wasnt reminded of Rikudou unless she saw Naruto using the same powers as him (floating) etc, Its probably then she decided to take their chakra, Why would she want to spend pointless efforts in fighting to absorb chakra of someone who might as well be a fodder