The question remains: why on Earth would Jesus privately give teachings to a man responsible for the death of countless Christians in the past, teachings that contradicted the ones he openly gave firsthand to his twelve disciples?

The answer to that question is that some of the greatest men of God have been some of the biggest sinners. Moses murdered a man in Egypt. David is said to have been a man after God's own heart, yet he commited adultery with Bathsheba and eliminated her husband by making sure he got killed in combat, which was an engregious abuse of his authority as king of Israel. Samson was a very flawed individual, yet he served a judge of Israel. Whether you believe such stories are true or are myth, the fact remains that in the context of the Bible God sometimes chose people who on occasion did some pretty bad things. Why are such individuals depicted as being chosen by God? I don't know. If the stories are myth, then such details are just plot holes. If the stories are true, then God's personnel decisions are above my pay grade.

(As an aside, many MAGAT Trumper thumpers apply the same logic to the Donald: he's flawed but God put him in the White House for a reason. I don't buy that for a minute, but I see where they're coming from. I think that if God has commissioned Trump to do anything, it is to destroy America. He's working hard towards that goal.)

As far as the contradictions between the teachings of Jesus and those of Paul, I'm not aware of any real contradictions. The biggest difference is a difference in emphasis. Jesus taught about the Kingdom of God, while Paul talked about Christ and him crucified. Those concepts are two sides of the same Christian coin.