"If I have spoken wrongly, testify to the wrong; but if I have spoken rightly, why do you strike me?" (John 18:23)

The guard who slapped Jesus' face did it out of  impatience; all Jesus did was only asking Annas in return, "Ask those who heard me what I said to them. They know what I said."

Whenever we lack patience, we would easily get mad, and will easily misunderstand others.
Jesus once said, "When someone hit your left cheek, give them your right cheek too”. But Jesus did not do that this time.  Was Jesus contradicting His own words?  I don't think so, Jesus’s original intention is to ask us not to take revenge as a means.

This time, Jesus’s approach was calm and gentle, and did not agitatedly fight back at the guard. This act could have influenced the guard so he will not badly treat another innocent person for the rest of his life.
What we need to learn from Jesus is: to always keep cool and calm.  In order to reach this goal, we need to memorise Jesus' word and act in our hearts. In time of need, the Holy Spirit will guide us.