Im not saying that they didnt have slaves but its not like God commanded them to be beaten or tortured. Continuing the example you gave of how slaves are to act towards masters the same chapter explains also how masters are to relate towards slaves.
Eph 6:9 You masters, do the same [showing goodwill] toward them, and give up threatening
and abusive words, knowing that [He who is] both their true Master and yours is in heaven, and that there is no partiality with Him [regardless of ones earthly status]. (amplified)
As for
Zechariah 14 i dont know how to explain it any better than
14 Behold, a day is coming for the Lord when the spoil taken from you (Jerusalem) will be divided in your midst. 2 For I will gather all nations against Jerusalem to battle, and the city will be captured and the houses plundered and the women ravished; and half of the city will be exiled, but the rest of the people will not be cut off from the city. 3 Then the Lord will go forth and fight against those nations, as when He fights on a day of battle.
Oh nevermind I see where you are coming from. verse 2 says "For I, (God) will gather all nations... "Now you can accept this or not. But Bible scholars agree that the OT is often to be understood in the permissive sense, not the causative. Meaning that v2 according to scholars and the fact that the Bible teaches free will it is understood that God isnt orchestrating people to go kill, plunder and rape the Jews but that it would be allowed. Zechariah is telling us that this will happen. Its just like how my mom as well as society taught us siblings not to drink and drive, but that didnt stop my brother from flipping his car cause he was driving drunk. I hope that explains a little. But even if it doesnt feel free to offer a few more examples showing God being in favor of raping infants etc. Just as the Bible tells its students we are to prove the Bible with 2-3 examples found in the Bible.
So again the Bible teaches mercy:
New International Version
"Woe to you, teachers of the law and Pharisees, you hypocrites! You give a tenth of your spices--mint, dill and cumin. But you have neglected the more important matters of the law--justice, mercy and faithfulness. You should have practiced the latter, without neglecting the former.
So yes the law is extremely sever and strict. You can read the 10 commandments and get an overall feel of the law and then theres quite a few more written out in detail. But mercy was always recommended. It reminds me of when the Pharisees (the jewish religious leaders) asked Jesus about the law, testing him. The law said the you could divorce woman for any reason. Jesus responded that that is true but not what god wants. Jesus states because of the hardness of humans hearts that that law was permitted. You can read about it in Matt 14.
Again the Bible always teaches mercy
If ye fulfill the royal law according to the scripture, Thou shalt love thy neighbour as thyself, ye do well: (James 2:8)
and later in the same chapter it says mercy rejoices against judgement.
Anyways if you have any more verses that you think are relevant please feel free to add them. Its ok to disagree Im just not convinced in the way you see things.