The passages about "vengeance" were not actually about vengeance. They were not commandments to individuals, but to be carried out by the rulers and authorities of the day. They were capitol punishments. The old testament states clearly to love our neighbor and not to take vengeance just like the new testament, and the new testament states that capitol punishment should be carried out by authorities just like the old testament. The ot and the nt are perfectly cohesive, they just have a different emphasis. http://bible.com/100/lev.19.18.nasb You shall not take vengeance, nor bear any grudge against the sons of your people, but you shall love your neighbor as yourself; I am the L ord . http://bible.com/100/rom.13.3-4.nasb For rulers are not a cause of fear for good behavior, but for evil. Do you want to have no fear of authority? Do what is good and you will have praise from the same; for it is a minister of God to you for good. But if you do what is evil, be afraid; for it does not bear the sword for nothing; for it is a minister of God, an avenger who brings wrath on the one who practices evil.