If Christians don't have to follow the laws of the Old Testament, why is the Old Testament part of the Bible?

I'm really sick of this back and forth here. Every time atheists point out the evil and barbaric parts of the Bible, Christians always come back and say, "That's in the Old Testament, so it doesn't matter anymore." OK, carpet scrapers, explain that one. If the OT has somehow been nullified,... show more I'm really sick of this back and forth here. Every time atheists point out the evil and barbaric parts of the Bible, Christians always come back and say, "That's in the Old Testament, so it doesn't matter anymore."

OK, carpet scrapers, explain that one. If the OT has somehow been nullified, why is it still in the Bible? Furthermore, exactly where does it say that the OT has been nullified? And if that is true, then doesn't that mean that one of the following must be true?

1. The god of the OT and the god of the NT are different gods.
2. The fact that a new set of rules was made proves god changed its mind, which means it is not infallible, which means it isn't really god.

It seems like millions of Christians only care about the OT and don't care about the NT at all. That's why so many Christians hate homosexuals, abuse their families, own stockpiles of weapons, and treat everyone different from them like animals. What is good about keeping the OT in the Bible if it leads to that?
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