Premanand Maharaj Ji
Why is criticising others so harmful to me spiritually?
Criticism takes on others' sins
When you criticise someone, you take on their sins and lose your own merit. So guard your speech, so that no criticism of anyone comes out.
That fool, by criticising another, takes all that person's sins on his own head, becomes a burden and goes to hell; the merit of many births is destroyed in a moment.