This is a common objection. Often we hear: "If the Bible isn't fiction, but you just said it isn't non-fiction, then what is it." Or "Either it's infallible or it's fallible, and if it's fallible, why pay any attention?" I personally believe there is a plenty of room for a middle ground.
The authors of each book of the Bible certainly were trying to influence their readers. But Christians believe they wrote in good faith. They wrote what they did because they felt God's word was being communicated through them, not verbatim, but in some underlying way. Many people have found this vague and abandoned the Bible, including some saints, like St. Augustine. But the bottom line is: the way we discern what the Bible is telling us is not by taking it as face-value. Every modern Christian is shocked when they read Leviticus just like atheists. So they have to discern what it means, and maybe they will decide not to focus on that section. Maybe it means God's people were willing to do whatever it takes to survive (just an idea, could be wrong). The Bible is huge, and few people have ever studied all of it, so instead Christian pray about certain parts and try to understand its meaning for their lives.
You might be comforted to know that priests often pray: "God I do not want my own desires and my own prejudices to influence my reading of the Bible. I do not want to have the ego to say 'I know what this means' right away." So they would not robotically read Leviticus and commit heinous acts, maybe they would try to read up on its history or pray about it, or consult a scholar.*
*To be 100% honest with you, I don't actually know the interpretative methods that Biblical scholars use. I kind of just trust that priests and scholars have arrived at reasonable conclusions of what is important and what is basically means. I understand the basic methods of interpretation and I defer to experts (sort of like how I know the basic tenets of the scientific method, so I trust scientists to tell me what's right without fully understanding every experiment). I understand that many passages are still in dispute and will eternally be up for interpretation, but I find interpreting the text to be a valuable exercise even if I change your mind about the text. It's safe to say it continues to be a topic of inquiry and interpretation, but few people would say we should use Leviticus to justify atrocities.