Is the fornicator who has been married, the one who kills a soul unjustly, and the one who abandons his religion and separates from the community, to be killed as a hadd (prescribed punishment) or for kufr (disbelief)?

The apostate is killed solely for his apostasy. The Prophet, peace and blessings be upon him, prayed over the Ghamidiyyah woman after she was stoned and supplicated for forgiveness for one who committed suicide. The generality of Allah Almighty's saying, "And if two parties of believers fall to fighting," and "And the two who commit it among you," indicates that the killer and the fornicator do not apostatize.