User blog comment:B214/Volume 21 info & spoilers/@comment-95.90.227.186-20160507145425/@comment-26345952-20160508030010

Look, the moment you accept an evil piece you are officially a devil for all eternity. But you will be a devil based on what you were before(human, dragon, youkai, half-blood, semigods, etc). So everything that you were, everything that you will born with will be intact, except your body is that of a devil, your whole existence is that of a devil.

The reason why gods cant be devils is because the evil pieces weren't meant to take gods and never will. One, they are so powerful and important that the wvil pieces could not match on them (not even if it was mutated), plus they are needed to maintain the balance of the world. Second the main goal of the devil is the ambition and greed of humanity.