When God put Adam and Eve in the Garden of Eden, they were both innocent and good. Yet, God allowed Satan into the Garden knowing that Satan would tempt Eve and that they would both end up sinning. Why did He do that?
On my radio show, an atheist called in and tried to trap me by posing a moral dilemma. He was smart enough not to appeal to his own atheistic moral standard, so he tried a different tact. He asked if I believed it was wrong to put a murderer and child molestor in a room with a child, knowing how bad the guy was and knowing what could happen. I said that it would be wrong to do so. He then applied that to God allowing Satan in the Garden and how that it is also wrong because God knew what would happen. Unfortunately, the criticism doesn't work. Here's why.
First of all, Adam was not a child. He was a grown man. He had--to some degree--knowledge of right and wrong because God had told him not to eat of the Tree of the knowledge of Good and Evil. In other words, Adam had been given a Law to follow that in itself included the option to obey or disobey. That is what the issue is about--not physical size in comparing a full grown bad guy and a small defenseless child. Adam made a moral choice, a wrong choice. So, the analogy doesn't work.
Second, let's work with the analogy and make it a little more accurate. Would it be okay to put a grown man in a room with a murderer? Why not if the man were strong and capable, etc? I've been in rooms with murderers when I did prison ministry. Was that wrong? No. Furthermore, all that happened in the Garden between Satan and Eve was a discussion. So, let's modify the analogy a little more. Would it be okay to put a murderer in a room with a man if the only thing that could happen was they were going to have a conversation? I see no problem with that--even if you know the bad person is very clever. But don't forget, to maintain the analogy, the "good" person knows full well what the right choice is supposed to be. So, he is not defenseless or helpless.
Third, applying my opinion about a bad man and a child to the issue of God's allowing Satan to talk to Eve does not invalidate the morality of what God did. The atheist would have to establish that it was morally wrong for God to allow the devil to tempt Eve. But how is he going to do that? If he appeals to the Bible for his standard, that won't work because according to the Bible, God does no wrong, and He has His reasons for allowing people to be tempted. If the atheist appeals to his own reason, then he's only expressing his opinion.
Fourth, God didn't cause Adam to sin. Adam freely chose--when presented with the fruit from his wife--to rebel against God. Adam fell because he freely chose to disobey God but second because he listened to his wife who had also freely chosen to disobey. It was a choice that they both made in full knowledge of what was right and wrong. It is not an issue of a child molestor in the same room with a kid.
Fifth, if someone doesn't like the idea that God knew they would be tempted and would fall and, therefore, says that it was wrong for God to let it happen, then what he would be requesting is that God not allow people to fall into sin, no matter what. Think about it. Freedom of choice means that temptations will occur. Robots with no free will are not tempted. They can only respond according to their programming. But Adam and Eve were different. Being made in the image of God, they had free will which--for them--necessitated the option of rebellion, an option that was provided by the devil. If it hadn't been he, it would have been something else later on! Furthermore, if a person says that God should have stopped Satan from tempting Eve, then shouldn't that same person be consistent and say that God should stop all evil? Why stop at with the Devil in the Garden? Shouldn't the person also want God to stop all murders, all rapes, all lies, and all evil thoughts? Shouldn't He then remove the ability from anyone and everyone regarding the free will choice to rebel and do what's wrong? After all, at what point do you want to stop and say this bad thing is okay to occur but that thing is not?
Finally, why did God let Satan into the Garden knowing what would happen? Because it was His will to do so. Just as it is our will to see how our children will respond to situations knowing that they will fail, we do so because that is what it means to grow, to learn, to exercise one's free will, and to take responsibility for our actions.