Originally Posted by LoathsomePete

One of the key differences that makes death a more accepted subject of dark humour is that it is an event that everyone will experience. Nobody is immune to it, and while it make come sooner for some who don't deserve it, and longer for others that do, in the end everyone will die. That universal fact makes it something that we will all think about at one point or another in our lives as is something we have to come to terms with.

I would say that subjects like rape, abuse, and genocide fall more in lines of what Frownland said about shock humour, or saying something that is so shocking that you laugh, not because you think it's genuinely funny, but to help assuage the awkwardness of the subject matter. Dead baby jokes are a perfect example of this. Not clever in any real sense or even witty, just take some horrible act and inflict it on something we are hardwired to protect and nurture.