Why is murmuring against God's promises considered evil?

Murmuring, as described in the Bible, is a sign of disapproval and distrust in God's plans. When the Israelites murmured against Moses and God's promise of a good land, they displayed an unbelief that contradicted their previous experiences of God's power. The evil report from the ten spies represented a rejection of God's ability to save and deliver. Thus, murmuring is fundamentally an expression of doubt in God's faithfulness and capability, which is why it is termed 'evil' in scripture.