In Mass Effect 2 it is possible to recruit Morinth into your squad by
Killing Samara instead of Morinth during her loyalty mission (Only possible with enough paragon/renegade).
With Morinth in your squad you can choose to romance her.
Even though she is an Ardat-Yakshi (They kill people by mating) she says that Shepard is strong-willed enough to survive mating with her. She mentions the Prothean beacon Shepard survived in ME1 as proof. When Shepard agrees to the mating, it will still end with death.
Was she lying?
If she knew Shepard would die I can't imagine she thinks she can get away with it. She is on a ship in space, together with all of the other crew. With Shepard gone they would surely get suspicious and find out. No way she could escape in the middle of space. Did she actually think Shepard would survive? Or is she simply insane and didn't think about it? She didn't seem very shocked in the end.
I assume this just is a little flaw they didn't really think about, since almost no-one will actually make this choice.