In the First Reading, God gives Moses and the Israelites the ten commandments. These are God’s basic instruction manual for life. In Christ’s Sermon on the Mount (Mt 5-7), that basic instruction manual is supplied with more details. The ten commandments forbid murder. The Sermon on the Mount commands you actually to love your enemy, turn the other cheek to him, and so on.
The trick is to see that the commandment about turning the other cheek has to be applied in the context of the broader rule: love your enemy. If you enable your enemy to become more evil, how have you loved him?
Elsewhere in the Gospel (John 2: 15), Christ uses a whip to drive the moneychangers out of the Temple. In doing this, he helps us understand how to apply the rules. In cases where turning the other cheek would make your enemy worse instead of better, loving your enemy requires helping him to stop his evil in some other way. And that is why the same Christ who gave the Sermon on the Mount drives the moneychangers out of the Temple. If he had turned the other cheek to the money changers, he would have been an enabler of their evil. But it is good for them, as well as for others, that they stop the evil they were doing. And so Christ uses force to get them to stop.
Here is the thing to notice, then. God’s instruction manual for life is not limited just to the sets of rules in the ten commandments and the Sermon on the Mount. There is a medieval saying: every act of Christ is a teaching for us (Omnis Christi actio nostra est instructio). Christ’s life and actions, as they are set out in the Gospels, are our best help for seeing how to live our lives well.Our best instruction manual is Christ himself.