I think that it's quite possible Aro might spare Joham. After all, creatures like hybrids would fascinate him, and maybe he could get hold of another child like Renesmee through Joham.
It's also possible that he might send Joham or his daughters to the Cullens to try and get Renesmee. Not to mention, I think, if Joham survives, he would want to meet the Cullens if he found out about them. He would probably view Carlisle as a scientist, like him, and as for Edward, who has successfully bred his own hybrid, well! I've said in another discussion that I often imagine Edward's reaction if Joham suggested he should help him in his experiments. I think Edward's response would be entertaining, don't you?