Both are killable in a normal run. For Mobius you just have to choose enough mean dialogue options before they show you the bottle of Gintegral, and then after showing you the Gintegral your options will be [what even is that?] and [that looks disgusting] (you'll get different dialogue options if you were too nice lol). Then regardless of what you choose afterwards, you'll eventually progress to [future you sounds miserable] or [*take away their drink*], the latter of which kills Mobius.
For Alexandria, when she asks if she can talk to you about her problems, choose [no] and your next choice should show up as [okay?] or [sounds obsessive]. Either will work, and same thing for the dialogue choice after that, and then you choose [you are being dramatic] > [I still think you're insane.] > [*grab one of the horns.*] which kills her :(