Who said they would die? God said so! The enemy here is now flat out calling God a liar. This is the same tactic he still uses today. Doubting the Word of God is not enough for the enemy. He wants you to dismiss God's Word altogether as being false. When Peter saw Jesus walking on the water he asked for the Word from Him to come to Him on the water. Jesus said come and Peter, relying only on the Word given to him, walked on water. The Bible says when the wind picked up and he was reminded of the storm around him, he took his eyes off Jesus and began to sink. Many a great sermon has come from this notion of taking your eyes off of Jesus. But imagine what was going through his mind at that time. At first, Peter just believed the simple word command of Jesus; "come." Then the enemy must have started to whisper in his head "look at the waves Peter! Feel how strong the wind is!" Peter began to doubt the Word Jesus had given him and shortly after that he simply stopped believing it. The result is that he sank into the water until Jesus rescued him.
Immediately Jesus reached out his hand and caught him. "You of little faith," he said, "why did you doubt?" Matthew 14: 31
What did Peter doubt? Peter doubted the word Jesus had given him when he commanded him to come to him on the water. The surroundings frightened him. His senses betrayed him. The enemy must have been whispering in his ear. That whispering first made him doubt and then ultimately to not believe the Word of God. That is the scheme of the enemy. To plant a seed of doubt in your mind about what God has said and thus gain a foothold. Once that foothold is established, the enemy seeks more territory and desires to expand to a stronghold by getting you to not only doubt, but also not believe what God has already said.
The last stage is that Satan desires to have you believe that you can take the place of God Himself:
"For God knows that when you eat of it your eyes will be opened, and you will be like God, knowing good and evil." Genesis 3:5
Remember, it was Satan who exalted himself above God:
You said in your heart, "I will ascend to heaven; I will raise my throne above the stars of God; I will sit enthroned on the mount of assembly, on the utmost heights of the sacred mountain. Isaiah 14:13
The devil wanted God's job but it wasn't available. He hates us because we are made in the image of God. He cannot physically harm us but he can lie to us. He can mix just enough truth to get us to doubt God. He can push those limits to try and get us to no longer believe what God has said. Ultimately, he wants us to believe that we can ascend to the role of God.