Matthew Henry Commentary Numbers 14:36-39

"And the men, whom Moses sent to spy out the land, who returned, and made all the congregation to murmur against him, by bringing up an evil report against the land, even those men that did bring up an evil report of the land, died by the plague before Jehovah. But Joshua the son of Nun, and Caleb the son of Jephunneh, remained alive of those men that went to spy out the land. And Moses told these words unto all the children of Israel: and the people mourned greatly." — Numbers 14:36-39 (ASV)

Here is the sudden death of the ten evil spies. They sinned in bringing a slander upon the land of promise. Those greatly provoke God who misrepresent religion, raise dislike in people's minds toward it, or give an opportunity to those who seek one to do so.

Justly are murmurers made mourners. If they had mourned for the sin when they were faithfully reproved, the sentence would have been prevented; but as they mourned for the judgment only, it did them no good. There is in hell such mourning as this; but tears will not quench the flames, nor cool the tongue.