God is traditionally understood to be perfectly good, omnipotent and omniscient. The existence of evil causes problems for this definition. If God is good, then he has the desire to eliminate evil. If God is omnipotent, then God is able to eliminate evil. If God is omniscient, then God knows that evil exists and knows how to eliminate it. But if God wants to eliminate evil and can eliminate evil, then why does evil exist? We can conclude that since evil exists, God – at least an omnipotent, good God – does not exist.
We can express this in terms of love. If you love someone, you don’t want them to suffer, and you will do what you can to prevent them from suffering. If God is all-loving, then he would prevent us from suffering if he could. Evil causes us to suffer, so if God is all-loving and all-powerful, he would prevent evil. Since evil exists, either God does not exist or God is not all-loving and all-powerful.