Christians must ‘turn the other cheek’ when slapped. Being slapped in the face is an insult, not attempted murder.
Nowhere does the Bible say that we are obligated not to defend ourselves or others against life-threatening attack.
Nor does it say that those who commit crimes should not suffer the appropriate penalty.
The crux of the matter is that there is a difference between public and private (or social and individual) ethics and responsibility.
Distinctly Christian ethics at the individual level command that we forgive—even the unrepentant. “Father forgive them, for they know not what they do.” Jesus’ pleading from the cross itself is our ultimate example.
However, public or social law requires that we punish evil doers. These (secular) laws should not be confused with godly morality or Christian ethics, as they only really cover misdeads that offend the public order. A man can hate and lust, and even lie all he wants (within limits), without breaking the law, but murder or theft or rape are public offenses, beyond being simply sins.
I’m always disturbed when I hear of a notorious criminal’s victim’s family bitterly saying, after a legal conviction, that “that’s the most he deserves!” as bitterness is never right...even if understandable. The obligation of the Christian is to personally forgive, even while upholding the law to prosecute and convict the offender.
In 3rd world countries though, this is often impossible—and even in ours (e.g. OJ Simpson) the punishment (if any) often doesn’t fit the crime. However, even when the legal system has perverted (public) justice, and we should do everything to prevent that, Christians must (personally) forgive.
“forgive us our trespasses, as we forgive those who trespass against us...” (from the Lord’s Prayer).