That´s why Satella/Emilia makes use of her "candidates". She transfers her witch powers over to somebody who she infatuates with herself, so that this candidate or rather servant then does exactly what she wanted him to do: Do everything he can to make he wish come true.
Since she´s a witch, there´s probably a fail safe buil-in, too.
And Betelgeuse shows us what a failed candidate looks like.