The risk that Shepard would be taking in this scenario, if she were to make the decision to save Liara's life at the cost of thousands of onnocent civilians, is very similar to what Liara found herself encountering when she handed the corpse over to Cerberus. That is, of course, that the person you love remains alive and well, but also hates you for your actions, despite the fact that you saved their life. Liara was absolutely terrified that Shepard would hate her for handing the corpse over to Cerberus, due to the fact that she wanted to see Shepard rebuilt. In the possible scenario that we are describing, Shepard would also risk Liara becoming disillusioned, or maybe even hating her in order to save her life, because she could not bear to have Liara die.
It's an interesting juxtaposition, and the nature of the choice that the person faces regarding their beloved is very similar.