“The very dust of Kurukshetra, borne away by the wind, shall cleanse persons of wicked acts and bear them to heaven!" The foremost ones amongst the gods, as also those amongst the Brahmanas, and many foremost ones among the kings of the Earth such as Nriga and others, having performed costly sacrifices here, after abandoning their bodies, proceeded to heaven. The space between the Tarantuka and the Arantuka and the lakes of Rama and Shamachakra, is known as Kurukshetra. Samantapanchaka is called the northern (sacrificial) altar of Brahman, the Lord of all creatures. Auspicious and highly sacred and much regarded by the denizens of heaven is this spot that possesses all attributes. It is for this that Kshatriyas slain in battle here obtain sacred regions of eternal blessedness. Even this was said by Shakra himself about the high blessedness of Kurukshetra. All that Shakra said was again approved and sanctioned by Brahman, by Vishnu, and by Maheshvara.'"
Shalya Parva Section 53 K M Ganguli’s Mahabharat
It made Kurukshetra a sacred battlefield, deflecting it from a dynastic conflict.
Arrogant and power-hungry, in his final moments, lying mortally wounded, Duryodhan declares he would go to heaven and mock the Pandavas. This reflects a belief in heavenly reward despite his actions.
Karna deeply believed in Dharma. He knew he had sinned (e.g., in Draupadi’s humiliation).
Kurukshetra being a holy land gave him hope that death would redeem him.
King Kuru’s boon, the safety net it provided shaped the ethos of Kurukshetra war — where even the worst of sinners could die as warriors and ascend.
Drona and Bhishma fighting on the side of Evil (the Kauravas) despite knowing the moral truth but deeply tied to their sense of duty and personal vows, were possibly aware of Kurukshetra’s spiritual boon.
The deaths of valiant Kshatriyas at the battle of Kurukshetra with the boon transforms the tragedy into a spiritually sacrifice.
picture from online source