There have been smart, disgruntled people forever. On the whole, I think they're less violent and therefore less dangerous than dumb, disgruntled people.
In terms of death toll, I'd probably disagree. It seems that 20 men from the upper classes (on horseback, armed and trained to use weapons) could destroy an 11th-century town of 1000-3000 inhabitants (although suffering heavy losses on their end) if they were so inclined. A powerful nobleman could assemble the knights.
In terms of impact, you're probably right. I think it's likely that 9/11 is the first news story to have reached half the world's population in a few hours. However, there are pre-modern events that rival it in terms of potential impact. I can't think of any from the 11th century, but the Defenestration of Prague comes to mind, as would the Gunpowder Plot had it succeeded.
If the Gunpowder Plot had succeeded, it wouldn't have been the destruction of parliament that would have been the issue, it would have likely been a full out campaign to kill every catholic in Briton by the most gruesome methods possible. The public reaction to the Gunpowder Plot was very 9/11 in terms of public opinion against the Catholics. In fact the Gunpowder Plot likely delayed the Catholic Emancipation by potentially two-centuries and increased intolerance towards Catholics quite dramatically.
Honestly if the Gunpowder Plot had succeeded, I have little doubt it would have turned into a massacre of Catholics in Briton and likely devolved into a full out war. Ireland would have received a full massacre for the guerilla war it had waged against Briton, and Spain would have likely come under full attack for supporting the Irish Catholics during that time.
It would be hard to see any result that isn't total war for a successful bombing. Ironically the plot was the worst thing to happen to Catholics but simultaneously the best outcome for the situation.