To me, it's the implication of KYS - that you should realise you have so little worth that you should take your own life. It absolves the speaker of the responsibility, the onus is on the one being spoken to.

Whereas DIAF is more circumstance. It's "I wish you an unpleasant death" rather than "realise how worthless you are and make the world a better place by removing yourself from it". One is more personal than the other to me, which is why I'd be fine with DIAF but not KYS. (I've never said either, for the record).