There are several reasons why an individual Muslim might go on a killing spree. Islam depends on the mercy of Allah for salvation. There's no guarantee unless one dies in jihad. Jihad has several meanings, from personal improvement to holy war. This kind of jihad is striving against unbelievers. Anyone who is an unbeliever or a hypocrite is a target. Who is a hypocrite? A secular Muslim, a non-practicing Muslim, or a liberal one. Many Muslims don't read the Quran or the Hadith. When they do they either are repulsed and confused by what they read, or they become more devout and serious about their faith. In Islam, prophets and martyrs immediately go to Paradise when they die; the rest have to suffer first.
"Not equal are those believers who sit (at home) and receive no hurt, and those who strive and fight in the cause of Allah with their goods and their persons. Allah hath granted a grade higher to those who strive and fight with their goods and persons than to those who sit (at home). Unto all (in Faith) Hath Allah promised good: But those who strive and fight Hath He distinguished above those who sit (at home) by a special reward." (Quran 4:95)
The UK takes in a large number of its Muslim immigrants from Pakistan because it's part of the (British) Commonwealth of Nations, a legacy of British colonialism. Tony Blair and the Labor Party was responsible for increasing their immigration. 84% of Pakistanis want sharia to be the law of the country. Only 28% of Pakistanis have a negative view of ISIS, 9% support and the rest are unsure. Why are they unsure? Because they can read for themselves that ISIS is practicing true Sunni Islam. Even the leading Muslim scholars at Al-Azhar University in Cairo, the leading center of Muslim academia, says they are real Muslims and won't condemn them.
Even more, 89% of Pakistanis favor stoning for adultery, and 76% believe that those who leave Islam deserve the death penalty. So if you're a Muslim who converts from Islam to become a Christian or an atheist, any Muslim can come after you to kill you. I personally know someone in the Philippines who left Islam to become a Christian, and she had to flee her home and stay in hiding. Her own brothers (who are police officers) want to kill her. (Some of the Philippine islands closer to Malaysia are Muslim-dominated, and some even have their own ISIS group called Abu Sayyaf.) A majority of Muslims in many Muslim countries believe this. If you want to look at the stats, look here:
http://www.pewforum.org/2013/04/30/the-w...s-about-sharia/