Murmuring or complaining is one of the sins that are overlooked quite often. A lot of times people go out of their way to focus on the "much bigger sins" such as murder and adultery that we overlook the sin of murmuring and complaining, but the Lord doesn't not over look it or give this sin a free pass. It is just the opposite. It's an unseen destroyer of the believer! The sin of murmuring and complaining started a plague that killed thousands of Israelites! [Numbers 16]
In this chapter, we read how the Israelites, who were to a great extent thirsty, looked only at the fact that the water they had come to was undrinkable because it was bitter. Look how fast they forgot God, who had brought them out of Egypt, from their slavery and led them out from there with signs and wonders. They had, not long before witnessed the Lord’s parting of the Red Sea and obliteration of those who had enslaved and imprisoned them. But they still, so quickly returned to looking only with their natural eyes at the problem in front of them. They were completely forgetting the miracle working power of their God! They murmured against Moses, but to tell the truth, when you stop and really look at it, they were really murmuring against God? Moses once again interceded for them, once again showing them the faithfulness of God. Instead of going to the Lord the Israelites would rather cry and complain. People are the same way today. Some things never change.