Why do liberals use violence if they disagree with policy?

[Jabber wock]

Non sequitur.

Your question asks about liberals, but your details refer to the Labour Party, who is in opposition to the government coalition that includes Liberal Democrats.

Indeed, the report you linked to quotes that Labour politician as tweeting:
"‘Well done our students – thousands outside the office getting stuck into the LibDem / Tory government.’

LibDem = Liberal Democrat. Don't you KNOW that?

So are you really trying to imply that 'liberal' thugs attacked an office used by the government that includes Liberal Democrats? Really?

The Deputy Prime Minister, Nick Clegg, who is a Liberal Democrat, strongly condemned the vandalism. It should be pointed out that the shadow cabinet, all Labour, ALSO condemned the vandalism, and so this article only refers to two individual politicians at odds with the bulk of their party. Furthermore, knowing the reputation of the Daily Mail, it should be taken with a rather large pinch of salt.

Even the article does not make clear if the Labour MPs were encouraging the vast majority of protesters who protested peacefully, or the thugs who were themselves also widely condemned by the genuine protesters. The organisers of the protest, the NUS, described the vandalism as 'despicable'.

The vandals are regarded as a hard core of anarchists, similar to those that have carried out similar thuggery at other events e.g. the G8 summit.

So none of the thugs were liberals, none of those who encouraged them were liberals, and indeed the target of the thugs included Liberal Democrats. Not only are you incorrectly identifying the motivation of the thugs, It seems you are incapable of telling the difference between liberals and the Labour Party.

Your question is nothing but a flawed attempt to denigrate innocents by wrongly attributing blame. Thus an apparent attempt at malicious denigration has been banjaxed by sheer incompetence.

Congratulations!