Joan Smith: Jail is no place for mouthy drunks. Ask Liam Stacey

Have you heard about the guy who posted a racist message on Twitter? No, I'm not thinking about Liam Stacey, the biology student from South Wales who's in jail – he lost his appeal on Friday – for a series of racist tweets following the collapse of the footballer, Fabrice Muamba. I'm talking about a young man from Manchester who tweeted his response to the 56-day sentence imposed on Stacey by in Swansea.

I can't repeat his tweet in a newspaper. But I can tell you that this young man expressed his ardent wish that Stacey would get "rapedddd" by "every black man" in prison. Other users of the social networking site demanded a longer sentence, advised Stacey, 21, not to bend over to pick up the soap – another jokey reference to male rape – and hoped he would drop dead in jail. The person who posted the last sentiment added the pious hashtag "pray4Muamba".

These people are not being prosecuted, to my knowledge. Their posts are as vicious, racist and sexist as Stacey's rant, and some of them could be read as an incitement to violence. The difference is that they have popular sentiment on their side, a circumstance the judge was conscious of when he sentenced Stacey for a racially aggravated public order offence. Judge John Charles said he was jailing the student to "reflect public outrage".

The judge also said the "whole world" was "literally" praying for Muamba's life at the time of Stacey's tweets, a claim as untrue as it is disturbing. I'd never heard of the footballer until he was stretchered off the pitch, and I was confident his survival depended not on God but the skill of doctors at the London Chest Hospital.

It quickly became clear to me that we were witnessing a Princess Diana moment, when compassion inverts its meaning in a wave of popular sentiment and becomes coercive. It was stupid and unfeeling of Stacey to crow over Muamba's cardiac arrest but the most appalling language is commonplace on social networking sites. If Stacey's tweets were a threat to public order – and it's hard to see how they could be, since no one has to read other people's tweets – so are thousands of messages posted by "trolls" who encourage hatred of women, gay people and foreigners.

A custodial sentence is wildly excessive and has worrying implications for freedom of expression, which is too important a subject to be brushed aside on grounds of "public outrage". Personally, I'm disgusted by the casual use of the word "cunt" as an insult but I don't think we should demand the arrest of every misogynist on Twitter. What we can do is challenge such people without engaging at the same level – trolls thrive on the adrenalin rush of invective – while trying to work out what fuels outpourings of abuse. In Stacey's case that could have been achieved through a community sentence, combined with the realisation that he has to live with the shame of his actions.

Instead, a young man's life has been wrecked and the trolls mocking him on Twitter are basking in self-congratulation. Sentimentality and cruelty are frequent companions, and feeling too often takes precedence over rationality in the virtual world. Alcohol-fuelled rants play a part but there's also a conspicuous failure of empathy and contempt for civilised debate. Not one of these problems has been solved by making a scapegoat of a drunken idiot.

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