WHY DID SHALYA DESERT KARNA?

The war between Karna and Arjuna had assumed importance since great warriors Bheeshma and Drona had come and gone. Moreover, Karna had promised his mother Kunti that he would only try to kill Arjuna and none else among Pandavas. Karna had also given away his Kundala to Indra despite being cautioned by his father Surya (Sun). Karna had the curse of Parashurama to make him forget the hymns at the most critical point of the war. Even this curse was the courtesy of Indra's son. After all this, Karna had promised his mother that he will reuse any Astra during the war. Shalya was highly self-centered, proud and not in support of Duryodhana. Actually, Shalya wanted to join Pandavas, when Duryodhana stealthily appeases and felicitates him on the way to Pandava camp. Shalya was an established Maharathi and Karna had not been established or identified as Maharathi. 

At a critical moment, the Sarpastra was set by Karna and he had aimed at the neck of Arjuna. Shalya knew the trick that Lord Krishna might definitely play. He advised Karna to aim at the chest so that any sinking that Lord Krishna might effect would ultimately slit the neck of Arjuna. Karna said: "I will not change the target once set." The arrow missed Arjuna's neck and removed the crown as Lord Krishna pressed the ground along with the chariot. Now, the arrow, which had a vengeance against Arjuna, returned to Karna and begged him to reuse it. Karna was unprepared. Shalya too forces Karna to reuse the arrow. When Karna refuses, Shalya, who was short-tempered, deserts Karna. That was the inescapable destiny.