Not sure what the downvotes are for (so have an upvote from me), because you bring up a concern any thinking person with a heart should have.
I really struggle with this issue too. There are a lot of issues we as Christians need to sort through in many OT passages. I don't have any answers, other than that theologians like NT Wright and Pete Enns along with some great black American theologians like Xavier Pickett.
Pete Enns did a book review of Derek Flood's book Disarming Scripture which seems to address a lot of this. I'm planning on reading it soon.
What I do know is that the entire Bible as a whole, and especially culminating in Jesus, points us to loving other people. And loving other people precludes slavery, which is horrific, oppressive, dehumanizing, and spits in the face of what God tells us, that people are created in his image.
There's no short answer to this. I recommend checking out one of the following books:
The Bible Tells Me So: Why Defending Scripture Has Made Us Unable to Read It and/or Inspiration and Incarnation: Evangelicals and the Problem of the Old Testament by Peter Enns
The Human Faces of God: What Scripture Reveals When It Gets God Wrong (And Why Inerrancy Tries To Hide It) by Thom Stark
There's a middle road between where your vector is headed vs. what you appear to think is vital to Christian faith.
On Old Testament violence and bizarre commandments, take a look at Derek Flood's Disarming Scripture.
On the prospect of endless hell, take a look at purgatorialism. A "great many" of the early Christians, and a growing number of us today, acknowledge that endless hell doesn't really make sense, and believe that purgatorial universal reconciliation may have the strongest Biblical case to make.
I wouldn't say it like "going against the very words of God" - its more like recognizing that within the Bible there are competing voices, and that we have to choose which ones to listen to. From a Christian perspective, Jesus commands us "Love your enemies and pray for those who persecute you, so that you may be children of your Father in heaven" (Mt. 5:44-45). I believe the phrase I put in italics indicates that being children of the Father means we love enemies, because that's what God does. Additionally, at the end of this passage, Jesus closes this teaching with "be perfect, therefore, as your heavenly Father is perfect." (vs. 48) It is interesting to me that this is a restatement of the well known Jewish statement "be holy, as I am holy", from Lev. 11:44 and Lev. 20:26. Matthew's restatement followed immediately after the "love your enemies" teaching, implying that perfection in this case refers to perfectly loving enemies. Additionally, Luke restates this same statement: "Be merciful, just as your Father is merciful." (Lk. 6:36)
I find this to be a complete contradiction with the vision of God in Joshua who instructs the Israelites to kill not only their enemies, but also the innocent women and children of their families. Is love actually displayed by killing those we love along with their families?
I think it's important to note that Jesus isn't doing anything completely and totally new here, either - there were competing Jewish voices in the "Old Testament" that taught non-violence. For more on this, I recommend "Disarming Scripture: Cherry-Picking Liberals, Violence-Loving Conservatives, and Why We All Need to Learn to Read the Bible Like Jesus Did".
So you see, it's not so much "going against the very words of God", as it is exercising critical thinking to decide which words really are "the very words of God", because we recognize that there are competing voices all claiming to be representing "the very words of God."