I have noticed a lot of insulting on Cathinfo recently. The most common insult I have noticed is the calling of other people idiots, or the equivalent. Is it a sin to insult someone like that?

There is not necessary an objective answer to that question.  Sometimes insults are hurled when one cannot defend his erroneous position, other times they are used in the hopes that the person will wake up and realize the insult is true and will try to change his ways.  It comes down to motive and intent.  But it is easy to convince one's-self that they are insulting a person for fraternal charity when in fact they are feeding their own ego.  I should know.  I do it all the time.  

When I hear someone insulted, I usually lose respect for the person doing the insulting, and not the person insulted.

 :dancing-banana:
Maybe.  It depends upon who is speaking to whom, about what, in which manner.  Obviously it was not a sin for Our Lord to call the Pharisees hypocrites or blind leaders of the blind.  Was it sinful of John the Baptist to call the Pharisees a brood of vipers?  St. Paul DID apologise for calling the high priest a whited sepulchre.  Was that a sin?  
Our Lord warns us about calling another a fool, and that every word we speak will be accounted for at our judgement.  In general, it is unbecoming of the Christian to insult others, but there are surely exceptions.  St. John is said to have insulted a known heretic and refused to be in his presence.  Is the intention to injure another's soul?  If so, it is sin.  The wise man inquires, "...a wounded spirit, who can bear it?" Is the intent to ruin another's good reputation by backbiting, detraction, calumny, slander?  This is spiritual murder.  Do we mock or scorn or reject God-ordained authority? Sin is not lacking.  Do we spout put-downs and crude remarks to amuse others or raise our sorry egos by lowering others?  Not good!  Do we speak the truth though it be hard to hear that another's soul be converted?  Though it backfire, it is not sin.  Do we warn others by way of strong words when it is our duty?  Again, not sinful, even if poorly-received.   Perhaps a test is this.  Ask if yourself if you would call another by a certain name to his face.  Ask yourself what this would accomplish.  
Cultural norms and social position must be taken into account.  It is not sinful for two old men to jokingly "insult" one another.  Both know it's a form of teasing.  It is another matter for an employer to put a negative performance review on-line to publicly humiliate without necessity.  Parents should not mock or scorn priests or religious in the hearing of their children, then lament their children's lack of respect for authority.
Does Bishop Williamson sin in referring to Bishop Fellay as a weasel?  Somehow, I do not doubt he'd say it to his face.  He doesn't personally hate Bp. Fellay, but he does hate his ambiguous and contradictory statements.  Bp. Fellay could reply in like words because they are both on the same level, two bishops butting heads.  If, as an ordinary lay woman, I walked up to Bp. Fellay and called him a weasel, it WOULD be sinful.  It is not in keeping with my place to do so, and could indicate only disdain for his spiritual position.  If I needed to convey displeasure, it must be done with respectful words.  On the other hand, with a priest whom I know well from childhood, I might make a sarcastic comment to tease him, and he might do the same to me!   If done at all, the context must be one where it will not be misunderstood and become an occasion of scandal to others.  Of course, we don't need to be "nithe to evweewon!"  Flattery can be a form of insult as easily as blatancy.  What greater insult to Christ than for Judas to greet Him with a kiss.  What could have been more generous than for Christ to then address Judas as "friend?"

I have noticed a lot of insulting on Cathinfo recently. The most common insult I have noticed is the calling of other people idiots, or the equivalent. Is it a sin to insult someone like that?

This forum is exceedingly polite, so you must be crazy!

When you have to recourse to personal attacks and insults, you have lost all argument. I think that uncharitable words and personal remarks do constitute a sin against your neighbor.

I think it is a sin. maybe it is not so much the insult though as the pride. pride is a capital sin.

I think instead of calling someone stupid, it is better to tell someone they are acting stupid. Instead of calling someone an idiot, say they are acting like an idiot.

Instead of telling someone you don't like them, tell them you do not like the way they act.

That makes things less personal.

Frances, the :dancing-banana: plays  :devil2:'s advocate!
Then Bp. Fellay is being charitable by separating Francis' words from his faith?

I can tell you this much, its a mortal sin and detraction to post on this forum that "Soulguard is mentally ill". I mean really, who are YOU? You think I ought to take this crap from some anonymous windbag who wants to post something insulting to someone without him knowing who sent it? Cowards. I seen a post saying that same thing and it got 4 thumbs up. It is obvious, there are some faceless cowards here on this forum, pretending to be Catholic, but treasuring their secular atheistic world and cracking down on people they dont like on the internet with the arsenal of atheistic weapons like psychiatry. It is the best insult you can make against someone to say they are mentally ill, and to idiots who dont know any better it looks to them as if you try to help the person, when in fact its just the purest insult possible. You people know nothing about me, nor are any of you qualified psychologists or psychiatrists. I would say that the person who knows most about this field is me out of all of you. I try to help people who you defecate on with your vain puffed up lifestyles of greed and pseudo superiority every day. An awful lot of people have an awful lot of respect for me. You will not take away my dignity with some cowardly abusive post here made anonymously on some internet chat room. Who do you think you are. I see a different person than the one you pretend to be, I see hypocrites. I will keep posting on this forum until I chose otherwise, maybe it is not worth my while talking to "pope" francis' hypocrites.