Those men who brought out an evil report of the land died by the plague before Hashem.
Torah Temimah: R’ Elazar ben Parta — Arachin 15a.
English: It is taught: R’ Elazar ben Parta said: Come and see how great is the power of evil speech, from the spies; if one who brings an evil name on trees and stones [is punished] thus, one who brings an evil name on his fellow — all the more so.
Kal vachomer from the spies to interpersonal speech.
Punishment for libel against the land proves the severity of libel against a person.