Why someone make dicussion about someone else ill manner?

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Why someone post discussion about someone else fault or misunderstanding on him/her discussion? If someone made mistake or maybe just misunderstanding on his/her post, isn't it better if we tell him directly our opinion to him? And not made another discussion about him/her without his known like this post! It's more like stabbing from behind...or maybe somebody just want the money? (*I won't reply any post, cause i hate this post)