Caleb's great question from the catechism: if God said don't murder, why did Jesus change the law. Was ok to hate your neighbor in the OT? Short answer - No. Love has always been the command. I can see why Caleb would ask this, and frankly, the way we talk about the OT and it's relationship to the new contributes to the confusion. We talk of an 'internalization' of the law, as though God didn't care about the heart in the OT. That couldn't be further from the truth! See, for example, the 10th commandment. The prohibition against coveting is clearly a heart matter. Moreover, Jesus' summary of the law (Matthew 22:36-40) is taken directly from the OT - Deuteronomy 6:5 and Leviticus 19:18,34).
Question #105: What is God's will for you in the sixth commandment?
Answer: I am not to belittle, insult, hate, or kill my neighbor— not by my thoughts, my words, my look or gesture, and certainly not by actual deeds— and I am not to be party to this in others; rather, I am to put away all desire for revenge.
I am not to harm or recklessly endanger myself either. Prevention of murder is also why government is armed with the sword (Gen. 9:6; Lev. 19:17-18; Matt. 5:21-22; 26:52; Prov. 25:21-22; Matt. 18:35; Rom. 12:19; Eph. 4:26; Matt. 4:7; 26:52; Rom. 13:11-14; Gen. 9:6; Ex. 21:14; Rom. 13:4)
Question #106: Does this commandment refer only to killing?
Answer: By forbidding murder God teaches us that he hates the root of murder: envy, hatred, anger, vindictiveness. In God's sight all such are murder (Prov. 14:30; Rom. 1:29; 12:19; Gal. 5:19-21; 1 John 2:9-11; 1 John 3:15)
Question #107: Is it enough then that we do not kill our neighbor in any such way?
Answer: No. By condemning envy, hatred, and anger God tells us to love our neighbors as ourselves, to be patient, peace-loving, gentle, merciful, and friendly to them, to protect them from harm as much as we can, and to do good even to our enemies (Matt. 7:12; 22:39; Rom. 12:10; Matt. 5:3-12; Luke 6:36; Rom. 12:10, 18; Gal. 6:1-2; Eph. 4:2; Col. 3:12; 1 Pet. 3:8; Ex. 23:4-5; Matt. 5:44-45; Rom. 12:20-21 (Prov. 25:21-22)).