Back in the bad old days of forty or fifty years ago, violent extremism was usually the province of revolutionaries, people who were so unhappy with the status quo that they were prepared to kill people in order change it. 

Nowadays, it seems that most of the violent extremists are defenders of the status quo; you might call them pathological conservatives. Islamic extremists perceive, correctly I think, that the flood tide of western materialism is undermining their "old-time religion" as relentlessly as it has undermined classic Christianity. The right-wing Christian extremists that are found mostly in the US have much the same perception and motivation. They see their religion, their culture and their whole way of life being eroded by secularism and tolerance of difference; where they can get control of the levers of government they react oppressively; where they are kept out of formal power, they react violently. This fellow in England seems to be more of same.