It would seem that Mark is attempting to summarize Jesus's central message to the disciples during this particular period, as he writes because he was teaching his disciples. He said to them, "The Son of Man is going to be betrayed into the hands of men. They will kill him, and after three days he will rise" (Mark 9:30). Over and over again Jesus spoke to his disciples of his death and resurrection.
It is quite evident that, while the disciples believed that Jesus was, indeed, the promised Deliverer, they continued to envision a victorious Messiah who would destroy his enemies and set up a temporal (earthly) kingdom. Jesus sought to rectify this misconception. This accounts for his apparent bluntness in referring to his coming death, They will kill him. Surely he could not put it more clearly than that. He was not at this time going to destroy his enemies with a vast show of power. On the contrary, they were gonig to seek to destroy him. The disciples understood his words but they did not understand the meaning of those words.
In this connection, Jesus used a title which he applied to himself quite often, The Son of Man. This title occurs fourteen times in Mark's Gospel. This can be seen as a reference to the humanity of Jesus, emphasizing the fact that, although he was the Son of God, he was also the Son of man, and, as a mere man, he would be subjected by his enemies to all manner of human suffering and even to death.
This title also seems to be an acknowledgment of Jesus's messiahship. The term was used by the prophet Daniel to describe one who would descend from heaven to preside over the last judgment and the new age (Daniel 7:13), which has Jesus looking beyond his death and resurrection to his coming again in clouds of glory.
Certainly, the disciples did not grasp any such implication as this. Perhaps they had at last come to realize that, in some way or another, dark days lay ahead, but they did not hold any assurance of the Lord's resurrection. It was a concept which, at the moment, was far too great for them to understand or perceive.