People are commanded to love God, one another and their enemies. This is the case today, and it was true in the Old Testament also. Yet it is clear that the death penalty was commanded by God many times in the Old Testament. The first and greatest commandment is to love God, and the Bible is very clear about how we do that: we keep His commandments.
John 14:15, 21, 23 and 24
(15) If you love me, you will obey what I command.
(21) Whoever has my commands and obeys them, he is the one who loves me. He who loves me will be loved by my Father, and I too will love him and show myself to him.
(23) If anyone loves me, he will obey my teaching. My Father will love him, and we will come to him and make our home with him.
(24) He who does not love me will not obey my teaching. These words you hear are not my own; they belong to the Father who sent me.
These verses are clear, and reveal the action model of the love of God, which is the biblical model of genuine love. Christ spoke very plainly and said that if we do what God commands, then we love God, and if we do not do what God commands, we do not love God. One of God’s commandments was that murderers be put to death, and it is not loving God to ignore what He said.
It is true that Christ taught us to love our enemies, but we must understand what he was saying when he said that. First and foremost, he was not contradicting his Father and the commands of the Old Testament. He was stating them in plain language. It was part of the Old Testament Law that people were to be loving, even to their enemies. Although many examples could be given, Exodus contains some very clear verses:
Exodus 23:4 and 5
(4) If you come across your enemy’s ox or donkey wandering off, be sure to take it back to him.
(5) If you see the donkey of someone who hates you fallen down under its load, do not leave it there; be sure you help him with it.
Since the teaching about being loving, even to someone that hates you, was a part of the Old Testament Law, we need to carefully examine the words Christ spoke.
(43) You have heard that it was said, “Love your neighbor and hate your enemy.”
(44) But I tell you: Love your enemies and pray for those who persecute you,
(45) that you may be sons of your Father in heaven. He causes his sun to rise on the evil and the good, and sends rain on the righteous and the unrighteous.
Christ said, “You have heard it said,‘Love your neighbor and hate your enemy.’” It is fair to say that most Christians think that it was God and the Old Testament Scriptures that said, “Hate your enemy,” and that Christ was changing the Old Testament Law. Nothing could be further from the truth. The Old Testament Law taught people to love, as we saw in Exodus (and there are many more examples beside those two verses). It was the religious leaders who perverted the Law of God and taught people to “hate your enemy.” The well-respected New Testament scholar, R. C. H. Lenski writes:
This [“hate your enemy”] is the way in which the scribes and Pharisees taught the people the second table. They mutilated even the words they quoted from Leviticus….This omission in the usual rabbinical teaching was no innocent abbreviation …This was a flagrant perversion of the law which included all the members of the Jewish nation down to the lowest and extended even to the stranger. 
It is imperative to notice that you can love your enemy and still obey the Old Testament Law, including executing justice and even going to war. Biblical love is not an emotion or a feeling, but is acting on what God commands. The words of Christ in Matthew make it plain that Christ knew that God loves His enemies, and even blesses them by sending them rain and sun even though they do not “deserve” His love and blessings. Exodus teaches us to be loving to our enemies and those who hate us by helping them out. The same God who teaches us to love our enemies in both the Old and New Testaments also commands the death penalty in both the Old and New Testaments.
It is love to obey God, and by having a swift death penalty we love the people in our society [many of whom are our enemies also] and offer them the best chance for a life free from fear and crime. By having a swift death penalty that is justly meted out, we are giving everyone the best chance to see the high value of life—theirs, and the lives of others. Without a swift and just death penalty, the value of life is cheapened. Most people in the United States, for example, live every day knowing that they may be killed by a drunk driver or by a robber in a convenience store. If they are killed, and if the criminal is actually caught, they know that their life was “worth” a few years in prison, if even that, because that is all that the criminal will have to pay. How can we call that “justice”? How can that teach the value of life? We assert that it cannot and does not, and that is a major reason the United States is overrun with crime today.
We are not loving God or people if we refuse to justly deal with criminals who will harm others. It is not loving people to allow an unsafe society to continue simply because we find it difficult to obey God’s laws. We are not really being loving if we allow murderers to go unpunished. The Bible says if there is no godly punishment of the wicked, then people who would not otherwise sin may be led into temptation, and that is clearly true. Just and quick punishments are a deterrent to evil behavior.
 R. C. H. Lenski, The Interpretation of St. Matthew’s Gospel (Augsburg Publishing House, Minneapolis, MN, 1943), pp. 244,245.