Why did the 10 spies bring back an evil report?
Why did the 10 spies bring back an evil report from the promised land after all they had seen, the signs and wonders that God had done for Israel?
To answer your question, .let us first find out who was the leader that was chosen to lead the spies through these 40 days of spying out the promised land.
According to the rabbinic teaching we understand that Moses blessed Joshua as the leader that was supposed to lead these spies as they spied out the promised land.
Moses blessing Joshua, this also means that he spiritually imparted the ability in him to make the right choices for that specific spiritual assignment.
Because of Moses' decision and because of the prophetic words that were going around the camp, that Joshua will become the leader to succeed Moses' leadership, these10 spies were in disagreement (spirit of jealousy) with Moses' decision of choosing Joshua as their leader.
I can picture them before me; gossiping and criticizing every spiritual decision that Joshua was trying to come up with, as they were on this mission of spying out the land, Joshua ends up having only one spy that was willing to follow his spiritual instructions and that was Caleb.
The 40 days and nights in the promised Land gave the 10 spies an opportunity to exercise their lustful desires and their unrestrained evil nature (they slept with the prostitute of the city and they also took a taste of every evil pleasure in the land) while Joshua and Caleb were doing their level best to stay faithful to the purpose of why they were sent into a place full of evil temptations.
Now, when they came back with an evil report to the people, it was not because they were doubting God, but it was because they thought that the people were not mature enough to resist the temptations in the promised land, and they also thought that it would be better to stay in the wilderness and continue to be faithful to God's laws rather than going into a land that is full of temptations that will eventually turn their hearts from keeping God's laws.
The mistake that the spies did was that they rebelled against God's blessing (the ability to make the right choices that was upon Joshua), and in so doing they resisted God's wise counsel and spiritual protection that was supposed to come through Joshua into their lives for these 40 days of spying out the land, because when you curse (when you resist or rebel against the spiritual leadership that God has put before you, this also makes someone's soul to repeat the mistakes and fulfil the evil desires in within one's soul).
Mankind is predestined to do evil and crazy things, if people refuse to acknowledge and honour the blessing of God that is resting upon the spiritual leader for that given generation.