This is not a mystery... Have you not seen the interviews of the Antifa and other leftist gun club folks who open carry at protests? They are fully supporting leftist candidates every one of which supports gun control. They aren't normal people packing heat for protection, they aren't gun rights advocates proving a point, they are people hoping to silence debate, to scare people into staying home, to keep the majority silent and unseen... same as racist Democrats are the first to point the finger at opponents with false charges of racism. The charge is intended to scare opponents into silent submission.
A mass shooting is just an extension of the same mindset, intended to use fear to silence the culture or normalcy the shooter rejects.