What is the turning point in the gospel of mark from a more positive public ministry to a focus on jesus death?

It is Peter's confession where he admits that  Jesus is the Christ and Jesus' revelation that the messiah must die.

In Christianity, the Confession of Peter alludes to a scene in the New Testament in which the Apostle Peter declares Jesus to be the Christ (Jewish Messiah). This is depicted in the three Synoptic Gospels: Matthew 16:13-20, Mark 8:27– 30 and Luke 9:18– 20. The Marks's verses are as below:

"Jesus went out, along with His disciples, to the villages of Caesarea Philippi; and on the way He questioned His disciples, saying to them, “Who do people say that I am?” They told Him, saying, “John the Baptist; and others say Elijah; but others, one of the prophets.”  And He continued by questioning them, “But who do you say that I am?” Peter *answered and *said to Him, “You are the Christ.” And He [b]warned them to tell no one about Him."