Fr. Lawrence Farley has an article at the OCA website about “Holy Hatred” in the Old Testament. His interpretation of what “hate” means in the Bible is pretty good. However, he makes an odd comment when criticizing an unnamed theologian’s view about violence and hatred in the Old Testament:
“Do I not hate them who hate You, O Lord? And do I not loathe them that rise up against You? I hate them with perfect hatred; I count them my enemies.” [Ps. 139:21] The author contrasts this attitude with Christ’s words about loving one’s enemies, and characterizes the voice of David in this verse as “the sinful voice of a human.” Though he says we ought not to “throw the Old Testament out, nor read it flatly without any discernment,” and though he asserts that while “Psalm 139 is full of inspiration,” he still says, “David’s own paradigm comes through. It’s all [David] knows in his time. He can’t yet apply the awareness of his divine belovedness [sic] to his enemies.” The upshot is that we must “pick and choose in the Bible. Always pick and choose Jesus.” That is, for him some bits in the Scriptures are devoid of inspiration or authority, and ought to be jettisoned since they are merely the voices of sinful humans, men incapable of rising to a divine standard. If something in the Old Testament mirrors the Gospel counsel in the New Testament, it may be allowed to stand. If not, out it goes. It is not the sinful Old Testament author’s fault however; “it’s all he knows in his time.” It is an extraordinary bit of exegesis, worthy of the heretic Marcion himself—or perhaps of the Biblical sceptics that made German theological liberalism so famous in the last century.
The author does not deny the inspiration of the Psalm, but states that it was tailored for its time. While the author may not speak perfectly about this issue, it is definitely harsh to link such a view to Marcion, who thought the Old Testament was evil. In fact, the author’s view might have much more in common with St. Theophylact of Ohrid than with Marcion. Consider Luke 9:52-56:
And he sent messengers ahead of him. On their way they entered a village of the Samaritans to make ready for him; but they did not receive him, because his face was set toward Jerusalem. When his disciples James and John saw it, they said, “Lord, do you want us to command fire to come down from heaven and consume them?” But he turned and rebuked them. Then they went on to another village.
Some manuscripts have James and John say “Lord, do you want us to command fire to come down from heaven and consume them as Elijah did?” This is because the narrative here clearly is in parallel with 2 Kings 1:9-12:
Then the king sent to him a captain of fifty with his fifty men. He went up to Elijah, who was sitting on the top of a hill, and said to him, “O man of God, the king says, ‘Come down.’” But Elijah answered the captain of fifty, “If I am a man of God, let fire come down from heaven and consume you and your fifty.” Then fire came down from heaven, and consumed him and his fifty.
Again the king sent to him another captain of fifty with his fifty. He went up and said to him, “O man of God, this is the king’s order: Come down quickly!” But Elijah answered them, “If I am a man of God, let fire come down from heaven and consume you and your fifty.” Then the fire of God came down from heaven and consumed him and his fifty.
Both narratives take place in Samaria, and the language is similar in the two. Furthermore, Luke 9:52-56 has parallels in Jewish and Christian literature where a harsh part of the Old Testament is rewritten in some way. Some may say that this passage is not intended to make Christ look superior to Elijah. There is something to be said for this, and it is how St. Augustine (Sermon on the Mount, I.64) and St. Bede (Expositio in Lucae Evangelium, ch. 41) interpret the passage (“The Lord blames them, not for following the example of the holy Prophet, but for their ignorance in taking vengeance while they were yet inexperienced”) but it is clearly not how Theophylact interpreted it. Luke 9:52-56 causes him to remark that the teaching of Christ is loftier than the life of Elijah (PG 123:829B, Explanation of the Holy Gospel according to Saint Luke).
All this to say that we should be slow to attack someone for their views, because they might not be so silly as we initially think (I have plenty of experience with this).
EDIT: more detailed information about the passage from Luke and similar ancient Jewish and Christian treatments of harsh OT passages can be found in Dale Allison’s JBL article ‘Rejecting Violent Judgment: Luke 9:52-56 and Its Relatives.’