Yet, the NT also has parts that say you shouldn't associate with nonbelievers and that also say you should kill them.

So? Let me know when they start actually following them.

Of course. Have you not figured out most Protestants only pay attention to the New Testament at Easter and Christmas. The rest of the time it is the Old Testament, an old Jewish book. It is not even their book! This is how stupid and evil many Christians are, reaching for the The Old Testament which is full of all the shit they really believe, like prejudice, misogyny, homophobia and more.