ANSWER: Briefly, before we answer your question into this pivotal Biblical event involving Isaac and Abraham, we need to note an important aspect of God's perfect character. Many times, His motives and reasons for carrying out a particular act (or not doing so) do not correlate with those humans would possess. Since he is all-powerful and the originator of all knowledge (Isaiah 55:8) His thoughts are far greater than ours are. Concerning the sacrifice of Isaac, we need to be careful not to judge God based on our own standards of right and wrong.
For example, from a strictly human (non-Christian) perspective, the sacrifice of Isaac at the hands of his father likely strikes most people as being unnecessary at best and at worse murder. The reason given Abraham for why he should carry out the death penalty on his son was not as punishment for some grievous sin he committed. Rather, he was simply commanded to take his life as an offering to the Eternal (Genesis 22:2).
Death is man's great enemy (1Corinthians 15:54 - 56) because, from a human point of view, it has a finality to it that we cannot overcome. We tend to find it especially abhorrent when, as it seemed in the case with Isaac, a person's life is cut short by the actions of others. This is one of many reasons why most societies severely punish those who murder and only allowing killing under special circumstances (e.g. war, punishment for certain heinous crimes, etc.).
Genesis 22 delineates the trying of Abraham's faith when he is personally commanded to sacrifice "his only son" Isaac by God (Genesis 22:1 - 2). He is told to carry out the offering on Mount Moriah. As an interesting side note, according to the tradition of the Rabbis, this sacrifice caused the death of Sarah. They believe she died, after Abraham left for Moriah, when she discovered her husband's true intentions. The Bible, however, does not back up this supposition.
Arriving at Mount Moriah, Abraham makes all the preparations necessary to offer his son to the Eternal. He makes an altar, binds Isaac, and places him on a pile of wood. As he lifts up the knife in order to take his son's life, an angel appears. The messenger from God not only stops the death but also reveals to us why it was required. Speaking for the Lord, he states, "Do not lay your hand upon the lad . . . for now I know that you fear God, seeing you have not withheld your son, your only son, from Me" (Genesis 22:12).
Though God knows "the end from the beginning" (Isaiah 46:10), He always allows humans to make their own choices. Although he knew what Abraham was likely to do, he still needed to test him to discover whether he would love him more than his son. All this foreshadows the Father, roughly two thousand years later, willingly choosing to offer up his only Son, Jesus Christ, as a sinless sacrifice to make our salvation possible.
Abraham had the faith to offer Isaac because he understood that God had the power, unlike humans, to resurrect him from the dead (Hebrews 11:19). All the great blessings that would befall his descendants, and the entire world, were made possible by this outstanding demonstration of faith (Genesis 22:17 - 18).