Karna possessed the deadly Vasavi Shakti weapon during the Kurukshetra war. He could kill Arjuna at anytime during the war.
But, did Krishna keep Karna's mind under illusion, so that he did not use that weapon?
Yes, Krishna did control Karna's and his associates' minds to some extent to protect Arjuna and himself as he clarifies to Satyaki when he posed the same question after Ghatotkacha was killed by Karna using Indra's Shakti weapon (referred to as the "dart" below):
Satyaki of prowess incapable of being baffled, asked the mighty-armed Krishna about the great car-warrior, Karna, saving, 'O Janardana, even this had been Karna's firm resolution, viz., that he would hurl that dart of immeasurable energy at Phalguna. Why, however, did not the Suta's son actually hurt it then at him?'
Vasudeva said, 'Duhsasana and Karna and Sakuni and the ruler of the Sindhus, with Duryodhana at their head, had frequently debated on this subject. Addressing Karna, they used to say, 'O Karna. O great bowman, O thou of immeasurable prowess in battle, O foremost of all victors, this dart should not be hurled at any one else than that great car-warrior, viz., Kunti's son, Partha or Dhananjaya. He is the most celebrated amongst them, like Vasava amongst the gods. He being slain, all the other Pandavas with the Srinjayas will be heartless like fireless celestials!' Karna having assented to this, saying 'So be it' (the desire of) slaughtering the wielder of Gandiva, O bull amongst the Sinis, was ever present in Karna's heart.
I, however, O foremost of warriors, always used to stupefy the son of Radha. It was for this that he did not hurl the dart at Pandu's son, owning white steeds. As long as I could not baffle that means of Phalguna's death, I had neither sleep, nor joy in my heart, O foremost of warriors! Beholding that dart, therefore, rendered futile through Ghatotkacha, O bull amongst the Sinis, I regarded Dhananjaya today to have been rescued from within the jaws of Death. I do not regard my sire, my mother, yourselves, my brothers, ay, my very life, so worthy of protection as Vibhatsu in battle. If there be anything more precious than the sovereignty of the three worlds, I do not, O Satwata, desire (to enjoy) it without Pritha's son, Dhananjaya (to share it with me). Beholding Dhananjaya, therefore, like one returned from the dead, these transports of delight, O Yuyudhana, have been mine. It was for this that I had despatched the Rakshasa unto Karna for battle. None else was capable of withstanding, in the night, Karna in battle.'
Dhritarashtra also asks Sanjaya the same question and he too replies that every night they would make up their minds to use the Vasavi Shakti on Krishna or Arjuna but they would simply forget about the weapon next day in the battle:
Dhritarashtra said, 'I see, O sire, that this act of Karna and Duryodhana and Suvala's son, Sakuni, and of thyself, in especial, hath been very much against the dictates of policy. Indeed, when you knew that dart could always slay one person in battle, and that it was incapable of being either borne or baffled by the very gods with Vasava at their head, why then, O Sanjaya, was it not hurled by Karna at Devaki's son, or Phalguna, while he was engaged with this in battle before?'
Sanjaya said, 'Returning from battle every day, O monarch, all of us, O foremost one of Kuru's race, used to debate in the night and say unto Karna. Tomorrow morning, O Karna, this dart should be hurled at either Kesava or Arjuna.' When, however, the morning came, O king, through destiny, both Karna and the other warriors forgot that resolution. I think destiny to be supreme, since Karna, with that dart in his hands, did not slay in battle either Partha or Devaki's son, Krishna. Indeed, because his understanding was afflicted by destiny itself, it is for this that he did not, stupefied by the illusion of the gods, hurl that fatal dart of Vasava, though he had it in his hand, at Devaki's son, Krishna for his destruction or at Partha endued with prowess like Indra's, O lord!'