Criminals, that's what they are. They place bombs in places where unsuspecting people are going about their daily lives. The bombs blow up, and people die. Criminals, pure and simple.
It's a tactic that's been used the world over. The Irish Republican Army, for example, used it widely, even in London. I can imagine French Resistance Fighters might have used this tactic in World War II (e.g. bombing cafe's frequented by Germans). We do know beyond the IRA, that the tactic has been used in lots and lots of countries.
As I think about this, any act of war incites terror. For example think of the scene in Saving Private Ryan where they come across a German family in their bombed out house, and the father is trying to give his daughter to the Americans as if that's the only way to keep his daughter safe. Think of how terrified the peoples of Germany must have been as the Liberating armies destroyed the German army. Past conquering armies have tended to slaughter the population wholesale, or rape and pillage. What would have been the fate of the German peoples? Fortunately we conducted that war with dignity, except for the firebombing of Dresden.
When a "terrorist" group like Al Qaeda uses the means available to it (lacking huge armies) and places bombs in public - of course they mean to incite terror. I remember listening to my friend, a jewish man from Brooklyn, who lives in East Jerusalem. He was in his final year of training in a spiritual healing program, and we were in a class that's a kind of group therapy session of sorts. I started to be aghast at hearing the vitriol he felt towards the Palestinians, and then I remembered where he's coming from, where he lives (in the land the Palestinians want for their capitol), and the depth of terror it must be to live with the constant threat of being bombed.
Organized armies, when they make attacks, also do so in a way that purposely incites terror in their "enemies". It doesn't matter that they do their acts under the approval of their government, and in their perception of the publics best interest. They are still terrorizing the people whom they are attacking.
And watch what the leaders say/do in the wake of these attacks. They mean to leverage the terror incited by the attacks to further their agenda, just as the "terrorist" organizations mean to leverage the terror to achieve a different agenda.
For example - GW Bush and Tony Blair and Donald Rumsfield and the rest of that gang all leveraged the September 11, 2001 attack to attack and subjugate a country which had nothing to do with the September 11, 2001 attack. Why Iraq? Iraq had nothing to do with it, it was the people holed up in Afghanistan, who have now been allowed to get away with murder.
What lies are going to be justified by the attack in London?