Probably when Victorians decided parts of the Bible were "mean". It's true that the New Testament supersedes the OT, but the Old Testament is not there to only fill up space, either.
In fact, many New Testament commands are right there in OT, too, including one of the most famous and important Love thy neighbour as thyself.
Leviticus 19 gives several commands on dealing with your neighbour starting with verse 13 and sums it up as follows (vs 18, KJV)
What does it really mean to love your neighbour? Modern liberal Christians will tell you it means being "nice" and accept him (her) as they are. Yet the very preceding verse states:
unconvinced of, unrepented of and persisted in, which may prove of fatal consequence to him; and therefore to let him alone, and go on in it without telling him of it, and reproving him for it, would be so far from acting the kind and friendly part, and showing him love and respect, that it would be an evidence of hating him at heart,