Almys is expecting to have his mind read. He did think about betraying her, once he was out of range of the Rope Trick. He decided it was relatively improbable that she was following him invisibly and reading his mind, especially as he went past advanced-looking mages on his journey. He could probably have gotten away with betraying her, if he'd felt like that. He is guessing that her magic can't kill him from a distance about that, based on a purely intuitive sense of which things she can and can't apparently do with her unaspected mana.
This left the obvious point that Almys doesn't want to kill the Summoned Hero and have the world end; and any course of betrayal that didn't end that way, would leave the Summoned Hero alive and powerful and pissed at him. It definitely seems preferable to end up as the near-immortal queen of a country instead. This continued to seem pretty obvious while he was sure nobody was reading his mind.
