Question: So if Adam wasn’t the first person to live and die, did God create men sinful because men only die because they sin? And if there were men before Adam they must have been sinful because they died.
Answer: Adam was told that the day he ate from the tree of the knowledge of good and evil would be the day he would die - "Of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall surely die" (Gen 2:17). But he didn’t die on that day, so God must have been talking about some other kind of death. Jesus said, "Whoever lives and believes in me shall never die" (John 11:26), but we still die a physical death so Jesus must have been talking about some other kind of life. That is the message of the Bible - humans, who are made in God’s image, can partake of the 'life of God' if they choose to do so.
“They are darkened in their understanding and separated from the 'life of God' because of the ignorance that is in them due to the hardening of their hearts” (Eph 4:18).
God didn't create us sinful, but he did create us with freewill. And freewill is exactly what it says: Free to do wrong as well as right.
When God first started to create life on earth, the ‘life’ started small. We have many fossils from those early days, and if we have fossils then it must mean the creature died. As ‘life’ got bigger, later in time, we have fossils for the larger life too, and if we have fossils then it means those creatures died too. God gives and God takes away. That’s the rule; a rule that applies to humans too. However, each human has an opportunity to grasp eternal life.
This universe was never meant to last forever; it is slowly running down as entropy takes its toll. The Apostle Paul said, "Flesh and blood cannot inherit the kingdom of God". It seems that humans were never meant to live in this universe forever. Even Adam was not going to live forever, he needed to eat from the tree of life to do that - After Adam disobeyed God's command, God said, “The man has now become like one of us, knowing good and evil. He must not be allowed to reach out his hand and take also from the tree of life and eat, and live forever.” This is important because we note that until Adam ate from the tree of life he didn’t have eternal life.
Adam wouldn’t have lived forever in the natural state that God had made him, he needed to eat from the tree of life to live forever, and it was in his natural state that God deemed creation, “was very good.” So physical death was encoded in Adam's DNA. "Precious in the sight of the Lord is the death of his saints" (Ps 116:15).
As we read through the first three chapters of Genesis we are taken by surprise at the end of chapter 3 when we find out that the “tree of life” is capable of giving Adam eternal life. We only get to hear that part of the story as Adam is being ejected from the garden. That’s the way God planned the drama that took place in garden of Eden. We don’t read that God told Adam or Eve to obtain eternal life while they were in the garden. God used what took place in the garden to teach us something important about the “life” he wants us to partake in, which is a far higher life than simple biological life.
The tree of life stood in a garden that God had planted. God told Adam he was free to eat from any tree in the garden including the tree of life (only the fruit from one tree was prohibited). But no one got to eat from the tree of life, or did they? Jesus proclaimed himself to be “the life” and said, “I tell you the truth, unless you eat the flesh of the Son of Man and drink his blood, you cannot have eternal life within you” (John 6:53). So maybe we can still eat from the tree of life, it’s not without reason that we are encouraged to “Taste and see that the Lord is good.”
See more on this subject in Is Death Adam's Fault?