It should be obvious by now, to anyone thinking, that American (and UK) Foreign Policy is deliberately designed to recruit more Islamic Extremists.

This policy is designed to increase religious hatred around the world, in order to increase the number of conflicts and wars. 
The increase in conflicts and wars is desired in order to increase arms sales around the world. 

The Woolwich killer, Michael Adebolajo, was :- 

British born of devout christian Nigerian immigrant parents 
He converted to Islam in his teens 
He became influenced by Islamic hate preachers after the invasion of Iraq 
He lived in a very poor area, with high unemployment and a large immigrant community and an existing problem of racist attacks

This does not in any way excuse the atrocity committed by Michael Adebolajo. 
It does explain why these attacks happen and why we should expect more of them to happen in the future, unless Foreign Policy is changed to stop exacerbating religious and racial hatred. 

The perpetrators of just about all recent terrorist attacks (and attempted attacks) have cited their principal motivation was American (and UK) Foreign Policy. 
This includes, Boston, the shoe bomber and London 7/7. 

Glenn Greenwald on the subject
http://www.guardian.co.uk/commentisfree/2013/apr/24/boston-terrorism-motives-us-violence

The foreign policies in question include :- 

The invasion of Iraq
The continuing occupation of Afghanistan 
The continuing occupation by 7,000 US mercenaries in Iraq
The drone killing of large numbers of innocent civilians, including women and children, in Pakistan and Yemen 
The use of double tap drone strikes terrorizing entire neighborhoods in Pakistan which increase civilian casualties 
The continuing use of extra-ordinary rendition for torture in places like Somalia 
The recent setting up of a US drone base in Niger 
The extremely large drone base now set up in Djibouti, Eritrea for attacks in Yemen and elsewhere 
The expansion of the US drone fleet

The smuggling of arms to Islamic Extremist rebels in Syria, by the CIA and/or State department, the condoning and support of the arming of Islamic Extremists in Syria by Qatar and Saudi Arabia, exacerbating that conflict and the atrocities being committed by both sides in the Syrian civil war 

The support of a puppet government in Mali which was installed by Western intervention in 2012. 

The support of a corrupt US sponsored government in Yemen. 
The use of cluster bombs and drone strikes by the US in Yemen, which are causing large numbers of innocent civilian casualties. 

The use of kill lists and targetted assassinations which are replacing old tired Islamic Extremist leaders with younger, more energetic, more hateful and more extreme leaders from amongst the multitude of new recruits to the Islamic Extremist cause. 

Pakistan has gone from 25% hating America 8 years ago, to 92% now. 
Primarily through the use of drone strikes. 
That is about 100 million new supporters of the Islamic Extremist cause. 

There are probably another 100 million new supporters of Islamic Extremists in the rest of the world. 
In countries like Egypt, who elected the Muslim Brotherhood, and Libya which is now run by an Islamic Extremist government.

Drone strikes in Afghanistan increasingly aid the recruitment of Islamic Extremists in that country, particularly when things like wedding parties are targeted. 
The continued support of an extremely corrupt US sponsored puppet government in Afghanistan, increases the support for the Islamic Extremists around the world.