Although the Gulf Arabians, Israeli Zionists, and their Pentagon sponsors are currently railing against the Persians of Iran spreading their "Shia Crescent" throughout the Middle East, back on Planet Earth the jihadist in the Manchester suicide bombing was a Sunni Arab whose family fled Libya when Qaddafi was in charge.
That means his family was a bunch of Islamist fanatics who couldn't tolerate the secular nationalist government of the Qaddafi era; the fact that many of them returned to Libya after Qaddafi was toppled, and jihadists had filled in the power vacuum, shows that it was not only this one guy who was an Islamist extremist in the family.
These are the people we are taking into our borders when we prioritize "refugees" from a country run by a secular strongman. That strongman is using force to keep the Islamists from exploding the country into anarchic jihadism, so those most likely to be fleeing such a country are those very potential jihadists. The Manchester suicide bomber's family was fleeing Libya under Qaddafi in the '90s, while future suicide bombers are fleeing Syria under Assad right now, incubating within America's and Europe's borders and destined to explode in the not too distant future.
While in Britain, the family must have been further radicalized by the extremist mosques in the country, most of which are run by the Deobandi movement, whose most infamous followers are the Taliban in Afghanistan. It is similar to the Wahhabi / Salafi extremism promoted by the Saudi government in the Middle East, which has given the world al-Qaeda and ISIS. The Deobandi movement comes from the Sunni branch of Islam in South Asia, where they are a substantial minority of the population (around 20% of Muslims), but exert an outsized influence in the religious schools (where they are about 60%) and religious militant groups (where they are about the only gang in town).
The Muslims of Britain are mostly South Asians, so this pattern is reproduced in that country as well.
Of the four schools of Sunni Islam, some are more prone to extremism, such as the Hanbali school of jihadist ground zero, Saudi Arabia, but also the Shafi'i school of Somalia and the Saudi-sympathizing region of Yemen. Those are the schools that urge female genital mutilation.
The Hanafi school found in the Near East and South Asia is relatively more moderate, but they are still prone to radicalization. The Muslim Brotherhood in Egypt, the Erdogan regime in Turkey, the al-Qaeda affiliates in Syria, Hamas in Palestine, and the Deobandi movement in South Asia, are all testament to how susceptible the Hanafi regions are to periodic radicalization, despite earlier periods of relative religious moderation.
Of course the schools that are naturally more extremist provide funding and other support to Hanafi regions in order to inflame the latent potential there, which has now stabilized into a Saudi-Pakistani axis of radical mosques.
Notably absent from the jihadist phenomenon is the Shia branch of Islam, and particularly Iran. Just go ahead and name for us all those Iranian suicide bombers in foreign nations, 9/11 hijackers, and invaders who convert others by the gun, and who destroy religious sites not favored by their narrow sect for being offenses against idolatry.
Hezbollah in the Levant is a national liberation militia, designed to expel unwanted foreigners by force. As long as unwanted foreign nations do not occupy their land, they don't care and don't attack them, let alone pro-actively invade to convert others to their sect, destroy monuments, and so on. They are defensive rather than invasive.
The same is true of the Iranian Revolution, which was designed to end its status as a client state of the US under the Shah. They did not forcibly convert or murder the non-Shia minorities, and during the 1980s they were at war with another Shia nation (Iraq), albeit one whose government was led by a Sunni. That was all about defending national sovereignty, not religious sectarian conquest.
There is something about Shia Islam that makes it less prone to intolerance toward other sects, proselytizing, revivals in the fundamentalist literalist puritanical direction, and attempts to radicalize members into jihadism. In outward behavior, it is more like the Catholic and Orthodox branches of Christianity, whereas the Protestant branch is more prone to periodic radicalization (even from members within the otherwise staid Mainline churches), with some Protestant churches being permanently radicalized.
That analogy is a topic for another post, but in the meantime have a look at a compare-and-contrast between Sunni and Shia Islam, topic by topic (you can ignore the third column). It's striking how Protestant the Sunni branch is, and how Catholic and Orthodox the Shia branch is, across a number of dimensions. I speculate that this stems from the Shia branch being deeply rooted around an early center of the religion, Baghdad, just as the Catholics are rooted in Rome, and the Orthodox in Antioch and Alexandria, the three early centers of Christianity.
The various Sunni schools are farther away from the eastern Fertile Crescent, and if they felt like "getting back to their roots," they would turn elsewhere back to the very beginning in Mecca and Medina. Similar to Protestants being rooted outside of the early Christian centers, and harking back to the Holy Land itself, and during Jesus' own time, for revival.
Going back to the very beginning unwinds all of the traditions that have developed in the meantime, so those groups without a root at the very beginning are less stabilized by tradition, and are more prone to radicalization.
Just a few initial thoughts, and something to explore at a later time.