No, you can actually beat her. You need to do quite a lot of damage in a limited amount of turns, but she is beatable. And then you get what you would have gotten if you had killed her right at the start of the game: The Vampiric Strike ability.
Interesting. There are so many different options and outcomes on the evil playthrough when it comes to Riliane that can easily elude you if you decide that the best option is to kill her right away. I didn't know about getting the healing spell for the slimes if you sacrifice her either.
Is it just me or you get way more freedom if you're evil? On the 0 corruption playthrough you're basically railroaded into doing things the only possible "right" way.