Hello Agnostic. Good to have you here.
I first want to say that I do not have all the answers, but that I do think I can help lay some framework on which you can think.
As was already stated, man was created in a perfect and sinless state, but he was tempted and fell into sin. Because of this sin, and the accompanying corruption and death, came upon all men. Romans 5:12. Because of this, man was separated from right relationship with God through spiritual death. Ephesians 21:1. (A side note but important. You are a spirit, a soul (mind,will, emotiont), and a body. When you hear a Christian talking about being "saved" or "born again", they are talking about a regeneration or a bringing back to life of their spirit which was dead in the sense of separated from God and from right relationship with Him.) The sacrifice that Jesus Christ paid was all about paying the price for sin and bringing us back into right relationship with God. God is Holy and just and a price had to be paid. Jesus did that for us, as we could not do that for ourselves.
But what about the time between Adam's sin and Christ? It is this time period that seems to you to paint a picture of a very different God than the God of Christianity that you find in the New Testament.
The Bible is divided into two sections that we call Testaments. Really, we could say these are the old and new covenants, or the way God dealt with man before Christ and the way He deals with man after Christ. It is the same God, just a difference in the way He dealt with man.
In the Old Testament you will find a lot of reference to the law of God. God chose the Jews (I am not totally sure why, but I accept it.) to be the people he would deal with and ultimately bring forth His Son. He instituted the law. This law is comprised of over 600 individual commandments and very harsh penalties. For example, backtalking your parents was punishable by stoning to death. No one could EVER keep these laws, and in the New Testament, we find that was the point of God dealing with man through the law. It was to cause man to despair of ever being able to be in right relationship with God by working really hard at keeping a set of commandments. You can find this in Galatians chapter 3. Paul the apostle told us that when the law of God came to him, sin revived in him and he died. In other words, he compared himself to the law, and it only made him aware of the power that sin had over his life. How could he ever please God by trying to live a good life according to the law of God? He could not. (Romans 7). God's original purpose was relationship with man. Man was created for His pleasure. The Old Testament, and the law, was a necessary step in bringing man back to right relationship with God.
The law of God was like a picture to man of how Holy God is and what His standards are. If we are to be right with God, we must be perfect in ourselves (impossible) or we must have a substitute, a savior. Jesus paid the price required by the law, and opened up the way that we can have right relationship with God through Him. He fulfilled the heart of God toward His creation. God's heart never changed, but the way he had to deal with man to bring about His perfect plan was different under the two covenants.
This is why God will be just in allowing people to go to Hell because they have refused to accept the free gift that was offered them of right relationship with God through Christ.
As to the question of murder, we have a wrong definition. Murder is a willful act of malice toward another. The commandment that said, "Thou shalt not kill." actually says, thou shalt not commit murder. War is a different creature entirely, and God did fight on the side of His chosen people. He also disciplined His people by allowing them to be decimated and defeated themselves when they did not obey and follow Him. When He told the Israelites to leave none alive, He was showing them the absolute purity He wanted among His people as a picture of the purity He desires in us.
I believe there is scripture to establish that all those who lived before Christ and the sacrifice he offered did ultimately have an opportunity to accept or reject Christ.
So, I hope that helps to clear up some of the questions. I would encourage you to continue to ask questions and remain open to answers. I know you said that you were trying to figure out if there is a God. I hope this does not sound too strange given that fact, but honestly ask God to show you if He exists. He wants to reveal Himself to you personally, not just academically.
Thank you for being open and honest enough to venture into Sermon Index and ask questions.