User blog comment:IamJakuhoRaikoben/Chapter 357: The Nine Demon Gates, Review./@comment-112.209.220.215-20131025141330/@comment-24066070-20131025181446

Considering the Nine Demon Gates were created by Zeref, it is possible that Silver is one of Zeref's latest or even the last demon created by Zeref and since Zeref at this point may already have been the soft-hearted guy who cares about human life and regrets his earlier lifestyle, he may have invented Devil Slayer magic based on his earlier encounter with Dragon Slayer magic and entrusted this magic to his last Demon in order have this demon (Silver) eradicate the other demons. Since Zeref may have been fond of humans at the time of Silver's creation, it could be he entrusted this demon with a fondness for humans, thereby explaining why Silver prefers his human form.