Christians don't get their morality from the Old Testament, they get it from the words and example of Christ. Christ on the Mosaic law:
"Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfill.".
" Master, which is the great commandment in the law?
Jesus said unto him, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind.
This is the first and great commandment.
And the second is like unto it, Thou shalt love thy neighbour as thyself.
On these two commandments hang all the law and the prophets"
And on the punishments prescribed in the Torah in particular:
"He that is without sin among you, let him first cast a stone.".
Christians are taught to hate the sin, but love the sinner: