The entire definition of Christian is someone who worships Christ. The New Testament.
Christ changes absolutely everything. His coming is a new deal, new rules, new morality, new reality.
Of course the New Testament contradicts the Old one. That's why they are different things. Think of it, as an American, as The Old Testament being English Common Law and The New Testament being The Constitution.
The old one may have some lingering effects, but the new one is the one that defines us.
A Christian follows the New Testament. Post Christ.
If you are troubled by the Old Testament and sacrifices and genocides and rapes and slaves then you'll need to take that up with Jews and Muslims.
For Christians, Christ is a brand new covenant with God. Nothing before is still in effect.