To answer your question, yes it is a sin to judge others and if you judge others you’re a sinner and will die, for the wages of sin is death (Romans 6:23)…
That came across as “judgey”, right? How much did you just want to change your actions to the better right after I said that? Probably not so much, which is a good reason not to judge and probably why Jesus used 5 verses in the books of Matthew and Luke to tell us that:
“Do not judge, or you too will be judged. For in the same way you judge others, you will be judged, and with the measure you use, it will be measured to you. “Why do you look at the speck of sawdust in your brother’s eye and pay no attention to the plank in your own eye? How can you say to your brother, ‘Let me take the speck out of your eye,’ when all the time there is a plank in your own eye? You hypocrite, first take the plank out of your own eye, and then you will see clearly to remove the speck from your brother’s eye.” -Matthew 7:1-5
Just because we know something to be wrong, doesn’t give us the right to judge, after all the position of “judge” is filled by Christ Himself (Acts 17:30, 2 Corinthians 5:10), and He warns us not to waste any time judging because we ourselves will be judged.
Judging someone rarely gets that person to want to change, on the contrary we all know stories where people were judged by so-called Christians and as a result no longer wanted anything to do with “Christianity”.
“It is impossible that no offenses should come, but woe to him through whom they do come! It would be better for him if a millstone were hung around his neck, and he were thrown into the sea, than that he should offend one of these little ones. Take heed to yourselves.” Luke 17:1-3
Jesus clearly states that people are bound to get offended by something, but warns us not to be the one to do the offence and cause them to stumble.
However, just because we shouldn’t judge, doesn’t mean we have to stay quiet it about things. The Bible encourages us to communicate – not in a judging way, but with patience and love:
“…reprove, rebuke, exhort, with great patience and instruction. ” 2 Timothy 4:2
“If your brother sins, go and show him his fault in private; if he listens to you, you have won your brother.” Matthew 18:15
“But encourage one another day after day, as long as it is still called “Today,” so that none of you will be hardened by the deceitfulness of sin.” Hebrews 3:13
At that point, it’s up to the person who we talk to to listen and change or else be stubborn about their ways.
“Do not reprove a scoffer, or he will hate you, Reprove a wise man and he will love you.” Proverbs 9:8