Oh man, death by starvation or dehydration is horrible. I think he actually didn't kill the original Cole kid and simply waited with him, having expected someone to come but it was too late when they did. Just waited and did nothing. Sat there holding his hand as he died, it would be a rather telling thing that Cole then decides to try and help the person by ending it quickly rather than letting them rot away like the original Cole had done. Lets them know there was someone there while they died rather than cold and forgotten.
He watches a young man die forgotten and alone and didn't do anything. Swallowed up by this horrible thing he witness, he tries to shove it away and forget it but he can't, it consumes him and so then the boy is never forgotten he takes his place. When he goes to those people so desperate to end it all, a familiar feeling of a long rotting end. They too know full well how horrible it will be and can't find a way to emotionally take it. There is only a bleak path and they send it out to the spirity/demon Cole we know, and absorbed and overwhelmed him again with the whole 'falling through the world'.
That was me just brain spewing what I thought about how that event could have been and why with Cole.