A large part of this violence seems to be really mindless - what psychologists call "infantile rage" - or a desire to "see the world burn" (to quote a line from a recent film). In infants, that sort of blind, irrational rage is normal but it is also normal to grow out of it. If infantile rage occurs in anyone older than infants, it's considered at best, unhealthy. If it occurs in an adult over time, one must then consider the possibility of anti-social or even psychopathic tendencies.
What is probably most disturbing to normal people is this beheading stuff. Execution by torture is considered barbaric by all civilized cultures and is only practiced by people who could only really be described as either insane or Satanic or both.
We have seen how leftists, most recently a far leftist "comedian" Kathy Griffin, affiliate themselves with beheading - as if it is somehow a funny or legitimate thing.
Just as troubling is the fact that in line with deranged or evil people/organizations that utilize terrorism to accomplish ideological/political objectives, there is no rational positive benefit associated with the use of terrorism - it does not achieve any success for those who employ it but rather, tends to be counter-productive to achieving any sort of political objective - i.e., it turns people who might otherwise be sympathetic to them away from them.
One would hope that such incidents such as Kathy Griffin's idiocy or this latest "beheading" event are simply spasms of acting out by disturbed people, not something that indicates a popular trend in leftists generally. But the lack of condemnation of such things or at best, a reluctant, diffident expression of what amounts to disappointment, really is cause for belief that it might be worse than that. Just as many Muslims tacit support and condone terrorism (roughly 10-30% by recent estimates based on polling of Muslims) there may be a large (and growing) contingent of leftists who also turn a willing blind eye to terrorist acts like this, because they are sympathetic to the causes which the perpetrators espouse.
In that case, if history is any gauge, they can expect a fairly major blowback politically and in general disapproval from the public as a result of the use of terrorist acts to further radical leftist causes.