The people murmured because their understanding of the nature of God was totally backwards: they said he hated them, yet he loved them as his children; they said he took them out of Egypt to hand them over to the Amorites, yet he delivered them in order to give the Amorites into their hands; they said God would destroy them, yet he wanted to give them life instead.
See the kinds of things God's people are capable of saying when their understanding of him is perverted!
27 And you murmured in your tents and said, ‘Because the LORD hated us he has brought us out of the land of Egypt, to give us into the hand of the Amorites, to destroy us.