The question is “Why did God command Abraham to kill Isaak and then rescinded this very command?"
As to the answer, it cannot be one-faceted, for it is such a dreadful mystery. Søren Kierkegaard's life-bewilderment cannot be an easy kernel to crack for anybody. Just to scratch a surface of the abyss-like mystery:
a) God wanted Abraham to learn that everything - his life, breathing, children, the love towards them, which is dearer for him than breathing, everything! - he has from the Giver of life, God. And so, nothing, not even love towards son must be put above the love towards Him who put this very love towards son inside your breast;
b) do not despair about future of your beloved ones, just do commandments of God, for He arranged it so that there is no death in the true reality contemplated by the Spirit-enlightened intellectual vision, for whoever dies physically while serving God, is not dying but goes from this physical death to life (cf. John 5:24), for an upright man survives his biological death and goes joyfully to God carried by angels, while his corpse is bemoaned by his relatives and friends; and even the physical death will eventually by the end of history be abolished by the general resurrection of all mankind and the souls of the deceased will reunite their resurrected bodies.
c) It is giving some mystical analogy of what it means that the Father did not spare His only-begotten eternal Son, but gave Him in order to save humanity: in a sense, all fathers and mothers should be like God-the-Father with relationship to their kids: at a certain moment the parents should realize that the child, ultimately, does not belong to them but is a gift of God and as such belongs more to God then to them, and the parents therefore should sacrifice their own vision on their son or daughter to that vision that God has towards him or her; to give a simple example, if a parent wants a son to become a banker in order to have safe and financially secure life, but sees that in divine perspective this son will and must become a musician, then the parent must, so to say, "kill" his vision of the "banker-son" and embrace the divine vision of the "musician-son", for the son belongs to Him who gave to him the talent of being a genius musician and "save" in a way humanity from listening bad or mediocre music like the Lord Christ saved humanity from the tyranny of sin.
The combination of the a), b) and c) is also possible and thousands of other no less plausible interpretations.