If an atheist is killed in this world, will he not be punished in the Hereafter, by analogy that the execution of a hadd punishment for a sin for which a hadd has been prescribed expiates the sin and its consequent repercussions?

The preponderant view is that the hudud (prescribed punishments) serve as expiation for their recipients, provided they die as Muslims. As for the disbeliever, he is not forgiven, for Allah the Almighty says: "Indeed, whoever associates partners with Allah, Allah has forbidden him Paradise, and his refuge is the Fire" and His saying: "Indeed, Allah does not forgive association with Him." Therefore, the hadd (punishment) for apostasy is a specific exception to the general expiatory nature of hudud. Thus, the killing of an apostate is not an expiation for him. Some scholars are of the view that the killing of an apostate is not a hadd but rather a punishment, and that the hudud are expiation for their perpetrators, whereas killing for apostasy is not an expiation, and its perpetrator is a disbeliever upon whom no funeral prayer is offered. The first opinion is stronger due to the mention of shirk (associating partners with Allah) in the hadith of 'Ubadah, but the generality of the hadith is qualified by Allah the Almighty's saying: "Indeed, Allah does not forgive association with Him."