He expected, ultimately, that Voldie would enjoy the opportunity for a little psychological torture--give a mother an impossible choice (stand aside, girl)-- if she does (unlikely) she'll hate herself, if she doesn't he has the chance to go back to Snape and say "I tried, but she wouldn't let me." Voldie loves to pull that kind of thing on his followers; look at his tormenting of Bellatrix at the beginning of DH. See my story "Betrayals" on sycophanthex/occlumency for a fuller explanation.