Why is it more of a question when a person receives a death sentence than
when a person dies a "natural" death? In either case further
self-improvement is not possible.
BTW, it is not accurate to say that through teshuva one is _guaranteed_ an
improved life in this world. In the world to come, yes, but there are no
guarantees to "the good life" in this world. To cite but one quick example:
A certain Talmudic sage suffered from extreme poverty and (to make a long
story short) Hashem told him that the *only* way that he could possibly
have an improved life in this world would be if Hashem would destroy the
world and start over again - and even then there was still the risk that he
would be poor!
Sorry, but there are no guarantees.