If this is the case, why did he need to replace his old laws with new love laws instead of just beginning with those laws?
Just because we are no longer that primitive. Did you ever play Civilization, any edition?
When society grows in experience, it starts to value others more than before. It's like when you teach your child - if he meets something hostile he should first go see his parents for it, while he's small, later on he'll be given some freedom in deciding, along with responsibility.
Why would God ever allow slavery to happen? Why would God ever allow children to get stoned to death for being unruly? Why would God ever want you to strike down your wife if she didn't know which God was real and had no way of knowing?
Well, do recall how people behaved at the time Abraham lived. Slavery all around, and "gods" of every way mankind thought. This is a sickness, but it's global, so to not ruin everything, we needed laws to first ease the dependency on slaves, then elimination when people will find out that their slaves are people too. Pretty much the same with all the other questions, including children - the pre-OT and OT society didn't value children.
Furthermore, do Jesus' new commandments cancel out the Old Ten Commandments?
No, they come along on the same level of priority. Note that "one superior commandment" that is "love God with all your being" and "love thy neighbor" are above the Ten Commandments in priority (actually it contains the whole of the first of the Ten in it). You claim "stealing of love"? You have to love everyone, not only those whom you plan to give the loot, but only that one who gets deprived of valuables by your hand. And, do recall the maxima "Do to others what you want to be done to you". Do you want someone else to steal from you?
If the Ten Commandments are still valid, then we have no way of deciphering what stays and what goes.
Well, we are given another scale to compare deeds against, that's why many of the smaller Old Testament commandments remain in place, but have zero appliance area, so effectively they cannot be applied anywhere. So technically everything stays, but as something cannot be applied to anyone, that something is irrelevant.
Why was God so cruel and active in the Old Testament and suddenly, now when things are verifiable, he's an invisible guardian that loves us but can punish us if we don't blindly accept him?
First, not "blindly" but "willingly". He is the only one who can guide you through death into vita aeterna. But first you need to learn how to live by words of God. Second, He is still active, you might at least read about Fatima's events.
It all depends on subjective morality; you want the Bible's morals to be in tune with society's morals so that we can all relate to it.
Instead, one will want society's morale to be in tune with the Bible's morale - this means, make society not sin as a whole. Sinning as a whole includes promoting free sex, abortions, drugs (narcotics, not medicine), divorces, child abuse (this includes juvenal justice), juridical corruption (as we have over here), and depriving people of homes, food, job opportunities (there are some places where one cannot find a job), wages below critical mark (country dependant, but if one works all day, he should earn enough for him to make a base living), and maybe more. Instead of sinning, the society should value life, family, charity and care for souls. However, the morale "if you don't want to work, then don't eat" should persist, to eliminate the problem where no one would desire to work but will live on the jobless funds.
^^ Well, this all is from a person who is no expert in politics, so other flaws could hide in this ^^
