People can turn to religion for a number of reasons. Commonly, they're uneducated, so they're more likely to be superstitious, they're grown up seeing violence around them, so they're desensitized to violence & feel that it's more acceptable, or, from time-to-time, don't see much of a point in life outside of religion.
Hence, how we see religious fanatics thrive outside of the developed world more often.
Back to the origins of terrorism, the 1st 2 videos here about modern-day terrorism are the most important. The others are just more in-depth.
"[Sunni] Islam is not a religion with a hierarchy such as the Roman Catholic Church. In that respect, Muslims are much like Southern Baptists, only more so. Any group of Muslims can build a mosque and hire themselves an imam, or teacher. They are independent. There is no Muslim pope or College of Cardinals. There are no bishops. When an imam somewhere issues a fatwa, which is a sort of formal opinion on a subject, it is not binding. Like Protestant Christians, Muslims interpret their holy writings themselves and consider themselves answerable directly to God – or, to use the Arabic word, Allah.Most of the disputes in the Middle East are secular and political. Hamas’ quarrel with Israelis is not about the fact that they are Jews but about the fact that they occupy Palestine. The objections some Muslims have to Western culture are the same as those some Christians have. They don’t like the violence, the immorality, the pornography and the greed. The conflict one sees between the religious and the secular in some Muslim countries is similar to the conflict in this country between religious and secular folks.There are 6 million or 7 million American citizens who are Muslims. Muslims have been in this country since the late 19th century. If you don’t already, you should make an effort to get to know Muslims. You’ll find that they don’t fit the stereotypes created by mean-spirited propaganda or superficial news coverage."
Jewish Defense League violence
Or do gun owners who use their weapons to unjustifiably harm others rightfully represent all gun owners?
Why won't the left criticize Islam? Are they scared to criticize Islam, do they hate western civilization, or something else?
I think that the left's reaction to criticism of Islam is a mix of a reaction to anti-Muslim & anti-Islam rhetoric, & Neoliberal virtual signalling.
Also, BareNakedIslam and Walid Shoebat are both openly pro-Israel & seemingly have an agenda to divide Christians & Muslims.
Since criticism of Islam from people who collectively hate Muslims often drain out the criticism from people who are not collectively hateful, a lot of people don't want to be associated with the virtual tar-baby topic of criticiseing Islam.
As for virtue signalling, they see Muslims as an oppressed minority group (though many of these same people only see this on a domestic issue, not foreign policy, basis).