In order to understand scriptures better you yourself would have to read them with a listening attitude all the way through full books, sympathetically trying to get what was the intended meaning.
This is quite incorrect. In order to read and understand anything, you must do it carefully and with full attention to context - and without bias.
As an atheist, I have no preference on whether God is a moral being or not; as far as I'm concerned, God is a character in a story. You can therefore rely on me to read the Bible fairly. If it turns out that God is a wonderful moral example, that makes no difference to me.
Christians, on the other hand, just believe that God is good, as it is an essential part of their religion. This explains why you are staring the facts of God saying "you can take, keep and punish slaves" in the face and denying them.
But, you'll perhaps possibly never get the full meanings, full imports, by listening to me.
We've already seen that. You, on the other hand, might get them from us.
But you still feel yourself it's the "worst forms", right?
RIght. Not the absolute nadir. That, perhaps, might capturing people against their will and torturing them for no reason. Much in the way that God sends people to hell. But "take people, keep them against their will, force them to work for you, and punish them as much as you like, just without killing them" sounds pretty close to the worst form of slavery in real life.
It seems you feel there should be more, immediately, at the start, right? I wonder if you are really asking:
Why didn't God say: "Do perfect Good all the time, in all things, always, and when you take slaves in war (instead of letting the defeated starve), only have them as slaves for a month only, and treat them like a brother instead of a servant"
Yes, that would certainly have been an improvement.
Aren't you really asking:
Why is there any evil at all in the world? Why wasn't it all ended long ago?
Not really, no. The Bible shows that God accepts an imperfect world, and His reasons for doing so are a different topic of discussion. However, it is you who said that God had a plan to eliminate slavery, and I am pointing out that you are obviously just projecting your own morality and preferences on to "history" as shown in the Bible.
Good point. If God really wanted to stop slavery, that is one of the things He could have done. But this is missing the point, which is that it God really was against slavery, He would have acted against it, and He didn't.
Christ uses exactly that -- beating slaves -- as representing evil that leads to hell (and there the extinction of the "second death") as the punishment after the Day of Judgement.
If you wish to read this verse as being against shipping slaves (which it obviously isn't) then we must point out that it is against slaves usurping the absent master's authority.
The passage means (and by all means look up a commentary to help you if you can't learn it from me) -- that God Himself will punish pastors/priests/church leaders of any kind on the Day of Judgement after this life if they hurt church members and others they are meant to help and care for.
As I thought. Therefore, it is irrelevant to say that it is against punishing slaves.
It's an example of the ultimate Law from God, today, for which the OP is asking: why isn't there Law against these things? -- Well, there is.
(sigh) If you are referring to the Golden Rule, it was obviously not effective. Slavery continued for centuries, and was eventually stopped by a complicated network of factors, among which Christianity was only a small part.
In other words, if God did have a plan, it was an extraordinarily ineffective one.
And so we come to the conclusion: if you think that God hates slavery and wanted to stop it, your only recourse is to picture a God of astonishingly limited powers and limited intelligence. Since that is not how Christians define God, we can safely say:
God supports slavery.