Just read you question again ... " How on earth can any religion hope to be taken seriously .... "
Well, when, "... stuff like this happens ...", I for one take it very seriously indeed, and I give very serious attention to Islam, at all times. It's far too dangerous to ignore. Consider:-
The Islamic atrocities of 9 / 11, the 7 / 7 attacks in London, the Bradford riots, the Paris riots, the Sydney riots, the Rushdie riots, the Madrid train bombing, the Mumbay train bombing, the Beslan massacre, the Bali bombings, the assassination of Robert Kennedy, the anti Christian genocide in East Timor, Southern Philippines, and Northern Nigeria. Add to this the attacks on Israel, and the threats to israel from Iran, and the killings of muslims by muslims in Iraq.
That's quite a list, and it's not exhaustive. So when hate preachers start calling for an overthrow of democracy in the west, particularly the U.K. , well, shouldn't we wake up?