God is the one who created human life as part of His great plan to show love to us. As Creator, He retains control over His creation, including the right to decide when to end someone's life. In Deuteronomy 32:39 God says He is the only true God and has the right to kill or to make alive.
God has also revealed that His plan includes resurrecting humans—bringing them back to life (something we humans cannot do). So even when God decides to kill a whole nation because of their gross sins and the misery they were bringing on themselves (as He did to allow Israel to inherit the land He had promised to Abraham), that is not the end of those people. He will bring them back to life later to teach them the right way to live.
"Taking another person's life is not our right to decide. That judgment is reserved for God alone. That is the thrust of this commandment. God does not allow us to choose to willfully, deliberately take another person's life. The Sixth Commandment reminds us that God is the giver of life, and He alone has the authority to take it or grant humans permission to take it."
All of these points support God's right to take human life, including using the Lake of Fire to destroy those who refuse His offer of eternal life. Though it might seem strange at first, even when God kills someone, it can be a demonstration of His love.