Firstinline, Jesus knew the rich man couldn't keep the law. The Law was a tudor leading to Christ. Only Jesus could fulfill the law. James 2:10 says, For whoever observes all the Law but makes a false step in one point, he has become an offender against them ALL." Jesus could have a called the rich man a liar but he didn't.
About turning the other cheek, Jesus didn't say anything about the third slap. Remember, Jesus did whip the hell out of the money changers in the temple, didn't he. Paul said, " ...as far it depends upon YOU, be peaceable with all me." Romans 12:18. Human imperfection gives us plenty of room for error and forgiveness.
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