1. Parshurama’s curse – Parshurama has cursed him so that he would not use the brahmastra weapon when he would need it most. So Karna would not be able to use this weapon at the most critical time. How would he expect to win the battle against Arjuna with this curse?

2. Brahmin’s curse – A Brahmin has cursed him that his wheels of chariot will be stuck in the ground. During Mahabharat war, when fighting with Arjuna, this thing happened. This caused him to step down from his chariot. Ramdhari Singh Dinkar has summarised this well in Rashmirathi.

“Jara si keech mein syandan phansa hai”

Above line and lines followed aptly describes the frustration. How a small mass of soft earth has held Karna’s chariot.

3. Indra’s manipulation – Confident of Karna’s charitable character, Indra approached him in the guise of a brahmin and asked for alms. Karna agreed to give him whatever he wanted. Indra asked for Kavach and Kundal to which Danveer Karna obliged. Ashamed of his activity Indra awarded him with Shakti. This weapon had the power to complete its task. The only caveat was that this could be used only once. Karna saved this for Arjuna. But Ghatotkacha, Bhima’s son, was creating havoc on Kaurava’s army. Upon the insistence of Duryodhana, karna has to use Shakti, thus saving Arjuna’s life.

4. Karna’s promise to Kunti – Kunti told Karna about his birth story before the Mahabharat war. She wanted him to fight the Mahabharat from Pandava’s side. Karna was the true friend of Duryodhana. He denied this request. Instead he gave a promise to Kunti. If there would be situation where I am able to kill any of the Pandavas, I would not do so except for Arjuna. In this way, Kunti will always have five sons.

5. Shalya’s reluctance towards Karna – Shalya, the charioteer of Karna never wanted to be on kaurava’s side. He never supported Karna during the Mahabharat war. Imagine this, how it is difficult when your navigator is not your side. On the other side, Shri Krishna was there to guide Arjuna.

6. Shri Krishna asking Karna for his righteousness : All over his life, Karna was the person, who was rightful. Despite all of the above reasons, Arjuna was not able to defeat him. Shri Krishna explained the reason to him. Goddess of rigtheousness was protecting Karna. How to defeat him because weapons were turning into garlands. Shri Krishna went to him as a Brahmin and asked Karna to gift his rigtheousness. Karna again, readily agreed and obliged Arjuna charioteer. Pleased of this, Krishna shown him his vishwaroopa. Shri Krishna also granted him a boon. Because of this, he was cremated on Shri Krishna’s hand.

7. Arjuna killing when he was not on the chariot: On advise of Shri Krishna, against the rules of the war, Arjuna beheaded Karna with the weapon anjalika. This  happened when Karna was trying to take out the wheel of the chariot from the ground. This is how, the great archer, the great danveer, met his death.

Perhaps destiny has to do more here rather than the capability. unfortunately he was fighting the war from the wrong side. Nevertheless he has got the everlasting fame.

After the death of Karna, the great, the Pandavas come to know of the truth from their mother, Kunti, that Karna was their eldest brother. They are at a loss how to react to this sudden disclosure of truth. When Yudhistra comes to know of it, he is so angry at Kunti for hiding this truth that he curses her and the entire women clan that they will not be able to keep any secrets henceforth.

The Pandavas don't shed a tear on the death of Karna as they have been thinking of him as an enemy since the time they knew him. Krishna asks the Pandavas, "Why are you all not shedding tears on the death of your eldest brother? Yudhistra, has your desire for the throne superseded your love for your eldest brother? Bhima, have you forgotten that Karna once defeated you and gave you a new lease of life in the war? Nakula, have you too forgotten that Karna also was soft on you and didn't kill you, even though he had an opportunity to do so? Yudhistra, have you forgotten that Karna did not shoot incessant arrows at you when you were without any weapons?" Yudhistra realizes his mistake and apologizes to Krishna, "Krishna, I now remember everything as to why Karna did not kill me. It was because of his promise given to Kunti that he spared me." Krishna explains to the Pandavas that Karna promised to Kunti before the war that he would not kill any of the Pandavas except Arjuna and that he would use the Nagastram (snake weapon) on Arjuna only once, although he had the option of using it on Arjuna multiple times. Arjuna too feels very guilty after listening to Krishna and exclaims, "How foolish I was! I tried to kill Karna, who was benevolent towards me and my brothers. I feel very guilty and ashamed of myself." Krishna tells Arjuna, "Arjuna, you always thump your chest and say that Karna is your birth rival and take pride in killing him, but do you know that you have killed a dead snake? Karna died not because you killed him. He died six times even before you killed him.

First, when a Brahmin cursed Karna that the wheel of his chariot would submerge when he was in an epic fight of his life.

Second, when Parusharam, his guru, cursed Karna that the Brahmastram (equivalent to today's nuclear weapon) that he learnt from him would not come in handy for him at the time he most wanted it and that he would forget it's usage when he most desired to use it.

Third, When Indra, asks Karna for his divine Kavach (the shiny armor embedded to his skin since birth) and kundal (ear rings) as a donation from him before the war.

Fourth, when Kunti asks Karna to promise her not to kill any of the Pandavas except Arjuna and also not to use the Nagastram on Arjuna more than once.

Fifth, when Shalya, Karna's charioteer, deserts him during the war at the most critical time.

Sixth, when I went to Karna in the form of a Brahmin when he was waiting for his death and asked him to donate me all the virtues accrued from all his benevolent and charitable acts since birth, including the virtue accrued to him by donating these at the time of death." 

Says Krishna, "These six incidents lead to his death, and it is not just you who killed him. Karna was invincible and no one could have ever defeated him. If he wanted, Karna could have easily killed all the Pandavas. So, Arjuna, stop feeling guilty for having killed Karna because his death was a combination of various factors and it was not just you, who were solely responsible for it."