Blah! Half-elf are more outcast. They can live in society but at the same time they aren't really 'accepted' so much. Its basically like the random minority in a primarily single race school. Sure you fit in and you can have friends and all, but in a way you still are a bit left out and not 'completely' accepted in. Granted that sounds racist but lets all admit the truth that happens. No matter what you say you always have a feeling towards someone who is different to you, race/religion/what not, no matter how much you accept them into a group.
Really though just used the whole 'outcast' idea of a half-elf maybe not finding true acceptance and simply deciding to venture into witchcraft which only would make that difference even stronger. :)