Mystique knew placing the baby on Rogue would cure her. There was a chance it would kill the baby, but it wasn't like that was her goal. And in the end, Mystique ended up saving Rogue's life and the baby was just fine. Would it have been better if she just sat on her hands and let Rogue die a horrible death?
And while Mystique's a murderer, when has she killed "countless tens or hundreds of other people"?