Who says that he can't help people on Tatooine?Luke says no one is really gone, so I don't think it should be as simple as punishment. It's doing what good he can, even if it will never be enough to justify the genocide of the first order, it's better than nothing, and it's more thought provoking than just an ironic punishment. For a series that has often had a black and white depiction of redemption and of a villain like Palpatine's being 100% evil, throw awrench in the middle and give the audience something to contemplate with. Even though SW is a classic story of simple morals, you can always add a new layer.
Ben can still be redeemed and punished without dying.