How is mocking others a mortal sin?
Glad you asked.
Saint Francis de Sales on mockery, taken from Introduction to the Devout Life.
“One of the worst conditions that a person can be in is to be a scoffer: God hates this vice extremely, and has in the past inflicted strange punishments for it. Nothing is so contrary to charity, and much more to devotion, as contempt and scorn for our neighbour.
Now derision and mockery can never be indulged in without this contempt; and therefore it is a very grave sin, so that theologians are right in saying that mockery is the worst sort of offence that we can commit against our neighbour by words, because other offences are committed with some esteem of him who is offended, but this is committed with contempt and scorn.”
Chapter XXVII
Of Propriety in words, and of the Respect Which We Owe to Others