They have tried raising them before remember? Frieren gave the example of the child demon that still killed it's adoptive parents and more even though it had been forgiven and such. That same story implied that humans have tried giving demons chances in the past, and that every single chance didn't work out. Not to mention there hasn't been a single demon that's talked about a "vegan/vegetarian" demon. With how long lived they are, that would have gotten around by now, especially for someone as long lived as an elf. At some point she would have heard of it too.

And every demon executed so far has been hostile, either attacking or already mid-hunt.