Is it possible that they left nothing, no word behind?
Did they make sure their phones would never be found or did they just play the game for their benefit alone?
Unless some sort of manifesto turns up, one can assume it was for their benefit. It meant something of , well.. as it turns out, of life and death importance. They died for 'it' . Whatever 'it' was, to them. They didnt want to live with 'it' , either. The burden of 'it' appears to have been too heavy for both of them to cart around , eventually. Apparently. It would take another determined killer to figure that out, and that's dicey, too. Since people are people, unique and individual.
There is a truism, people do things because they enjoy it. Now.. yes, people do terrible things, to themselves, cutting, burning etc, but the bottom line is, it provokes a certain joy. Not as I see joy, and probably not as you see it, but joy, exhilaration, none the less.
So it has to be assumed that these killings were rewarded, if I may put it that way, with exhilaration. For each other, and to each other. And particularly, not for the benefit of anyone else at all. At the very least.