What you did was serious; in fact, almost nothing is more serious in the eyes of God (and of society) than deliberately taking an innocent person's life. The Ten Commandments state it clearly: "You shall not murder" (Exodus 20:13).
But there is only one sin that God cannot forgive -- and that is the sin of refusing His forgiveness. Yes, you committed a great sin -- but you would be committing an even greater sin if you rejected God's forgiveness. And that is what He offers you! You may hate what you've done; you may even hate yourself for doing it. But God still loves you, and He yearns for you to come to Him and be with Him in Heaven through all eternity.
How do I know this? One reason is because your acquaintance was right: Jesus Christ even forgave a murderer. Who was it? It was Saul of Tarsus -- who became the Apostle Paul, the greatest Christian who ever lived! At one time Saul hated Christians, and was responsible for sending many of them to their death: "I persecuted the followers of this Way to their death" (Acts 22:4).
But God forgave him and changed his life -- and the same can happen to you. Don't carry your burden of guilt any longer, but by faith turn to Jesus Christ and give it to Him. Then ask Him to come into your life and cleanse you of all your sins -- and He will.