Jesus agrees with God and his word, but he does not agree with the way the religious leaders have applied God's word. The "I" is emphatic. This indicates that what Jesus says is equally important to the original commands from God. Try to translate this phrase in a way that shows that emphasis.
To strike the side of a man's face was an insult in Jesus' culture. As with the eye and the hand, the right cheek is the more important one, and striking that cheek was a terrible insult.