Since demigods are allowed I propose Herakles (accidentally) killing Kheirôn/Chiron.
Kheirôn was the immortal son of the Titan Kronos and therefore a half-brother of Zeus.
When Herakles was battling members of the Kentauroi/Centaurs of Mount Pholoe he accidentally wounded Kheirôn. The problem was that the arrow that pierced Kheirôns was poisoned with Hydra venom and as such was incurable. Suffering unbearable pain Kheirôn voluntarily relinquished his immortality. After relinquishing his immortality Zeus turned Kheirôn into the constellation Centaurus or Sagittarius. There is some disagreement which of the constellations represents Kheirôn. Eratosthenes identified the Archer not as a centaur but as the satyr Crotus, son of Pan, who Greeks credited with the invention of archery.
"Herakles (Heracles) turned back [the Kentauroi (Centaurs) of Mount Pholoe] with a volley of fire-brands; he sent arrows after the others and chased them as far as Malea. There they took refuge with Kheiron (Chiron), who, after the Lapithai (Lapiths) had driven him from Mount Pelion, settled on Malea. Herakles let loose an arrow at the kentaroi as they huddled round Kheiron, which penetrated the arm of Elatos and landed in Kheiron's knee. In horror Herakles ran to him, pulled out the arrow and dressed the wound with a salve that Kheiron handed him. The festering wound was incurable, however, and Kheiron moved into his cave, where he yearned for death, but could not die because he was immortal. Prometheus thereupon proposed Herakles to Zeus, to become immortal in place of Kheiron: and so Kheiron died." Pseudo-Apollodorus, Bibliotheca 2. 83 - 87 (trans. Aldrich) (Greek mythographer C2nd CE)
An excerpt from Diodorus Siculus' Library of History from the 1st Century BCE:
"[As Herakles was slaying the Kentauroi (Centaurs) of Mount Pholoe with his arrows :] Herakles unwittingly by a shot from his bow killed the Kentauroi Kheiron (Centaur Chiron), who was admired for his knowledge of healing." Diodorus Siculus, Library of History 4. 12. 8 (trans. Oldfather) (Greek historian C1st BCE)
A 17th Century representation of the constellation Centaurus showing the name Chiron in the constellation of Centaurus.
A 9th Century representation of Sagitarius featuring Crotos.