What did Pontius Pilatus do wrong?

Pilate was a victim of his own, transactional ethics. He had a golden opportunity to become a loyal follower of Jesus Christ, but he chose not to. Thank God!

He caved into mob pressure. In other words, he wimped out. He tried Jesus and found him innocent. It was his job to protect the innocent.

 Much like cocaine Mitch and several others …..Political expediency

"Then Pilate announced to the chief priests and the crowd, "I find no basis for a charge against this man" (Luke 23:4)

"When Pilate saw that he was getting nowhere, but that instead an uproar was starting, he took water and washed his hands in front of the crowd. "I am innocent of this man's blood," he said. "It is your responsibility!" (Matthew 27:24).

He "should" have kidnapped Mary Magdalene and sold her 

to the highest bidder, and then burned Jesus at the stake. 

Even back in the day, hell could easily have accommodated 

another moron who claimed to be a messiah, and the son of God.

If we accept the Gospel accounts

then what he did wrong

was acceded to the mob demand for Jesus' execution

rather than "taking the difficult road" and releasing him.

The Pharisees were rich money lovers as is most religious leaders who are inn bed with politicians.  So they were in bed with the Jews who shouted for Jesus death.  So it was what was prophecy would happen but do we see any different today.  The religious leaders are rich and send young men into endless wars for the political leaders to gain power and riches.

Gave in to the will of mob mentality. It wasnt like today though, rebellions were real threats and if roman officials could not stop them, even they may be executed.