I plan to gather more examples. But, one that really stands out to me:
In the Sermon on the Mount, Christ taught us to love our enemies. I listened to a sermon by David Guzik, and if he's correct, this love goes beyond emotion (e.g. care for their soul) and extends to even assisting (though not assisting in evil acts) not just personal enemies, but enemies of society. So, for example, we might assist a serial killer in his walk through life.
This seems very contrary to the Old Testament, which has imprecatory psalms and in which Ecclesiastes tells us that there's a time for hate.
I'm curious about this specifically, but I'm also curious generally: why does the NT seem so different from the OT, at least when it comes to ethics?
The NT and the OT are the same in regard to ethics and Jesus did not teach anything that was brand new in the Sermon on the Mount. Whenever he directly quoted Scripture, he proceeded by saying "it is written", but when he was quoting from what the people of his day had heard being taught about the Law, he proceeded by saying "you have heard that it was said", so his emphasis on the different form of communication is important. So Jesus was not sinning in violation of Deuteronomy 4:2 by adding or subtracting from the Law, but rather he was fulfilling the Law by correcting what was wrongly being taught about it and by teaching how to correctly understand it as it was originally intended. For example:
Matthew 5:43 “You have heard that it was said, ‘You shall love your neighbor and hate your enemy.’
While the OT certainly instructs us to love our neighbor (Leviticus 19:18), it does not instruct us to hate our enemy, so that is what Jesus was correcting. On the contrary, the OT says the opposite:
Exodus 23:4-5 “If you meet your enemy's ox or his donkey going astray, you shall bring it back to him. 5 If you see the donkey of one who hates you lying down under its burden, you shall refrain from leaving him with it; you shall rescue it with him.
Deuteronomy 23:7 “You shall not abhor an Edomite, for he is your brother. You shall not abhor an Egyptian, because you were a sojourner in his land.
Proverbs 24:17-18 Do not rejoice when your enemy falls, and let not your heart be glad when he stumbles, 18 lest the Lord see it and be displeased, and turn away his anger from him.
Proverbs 25:21-22 If your enemy is hungry, give him bread to eat, and if he is thirsty, give him water to drink, 22 for you will heap burning coals on his head, and the Lord will reward you.
So what Jesus was teaching was in accordance with the OT. Part of coming to love people that we once considered to be our enemies is recognizing that they were never truly our enemies:
Ephesians 6:12 For we do not wrestle against flesh and blood, but against the rulers, against the authorities, against the cosmic powers over this present darkness, against the spiritual forces of evil in the heavenly places.