Opp to what shown in TV, Karna had no such opportunity to kill Arjun on 16th day of war. Infact it was Arjun who made whole Kuru Army including Karna retreat from battle. Screenshot one and two of the incident and read 3rd and 4th Dhritrashtra and Karna's comments after retreat 
Even BORI agrees to same thing.
At that time Karna shot that Nagastra he was warned by Shalya that he won't be able to strike head with this. Shalya was a great charioteer , warriors used to compare him with Indra's charioteer Matali but Karna ignored his advice. That arrow was actually Ashwasena snake.
KMG : Karna didn't know it was Ashwasena who had personal enmity with Arjun since Khandavprasth Dahan , plan was to jump on Arjun via Karna's arrow and bite him. As a charioteer it was Krishna's job to protect his rider from that snake bite and he did so.What's wrong in that ?
As an excellent charioteer Krishna made horses bent their knees. Arjun asked his help as a charioteer only, before war, not his army neither weapons. He didn't even want him to fight. Why do you think he did so ? 🤔 So he can watch him die via snake bite ?
BTW Karna's charioteer also saved Karna by negotiating with Bheem. :) @Ojha_Mayankk
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