Keshav tried one last time to convince Karna to join Pandav's shivir. Keshav's rationale was Karna would be sitting on the sidelines during the yuddh b/c of Bhishnma's condition for accepting the senapati position.
Recap: Bheehsma's condition was that Karna won't take part in the yuddh as long as Bheeshma is alive. Bheeshma has ichhamrityu boon so chances were Karna won't get to fight at all.
This attempt was at the eve of war knowing fully well that the war won't be averted. Keshav still tried his best to get Karna on Pandava's side--- the Dharma's side --- knowing fully well what all Karna said and did so far. It shows Keshav did not consider Karna to be adharmi. Why only Karna - not all who fought from Kaurav's side were adharmi.
Keshav knows everything. To assume anything else is ignorance.
Treat the above situation that you described as Keshav giving karna one last opportunity to redeem himself. And he gave that opportunity to everybody else that he had talks with in kaurav's shibir on that eve. Matter of fact, Yudhistir does that too right before the yudh starts. This might sound cliched but it's something like being able to lead a horse to water but not being able to make it drink.
Once the war started, Keshav made things abundantly clear to those still unclear or unsure.