Another part of this is just old fashioned anti-Semitism. When my own synagogue was firebombed in 1999, it joined a grand tradition of physical, legal, and rhetorical violence against Jews with roots across the limpeza de sangre (blood purity) laws of 15th century Spain straight through to an April 2019 New York Times editorial cartoon that superimposed Benjamin Netanyahu's head wearing a Star of David necklace on the body of a dog being walked by Donald Trump. Anti-Semitism posits that Jews are "a distinct, inferior and troubling race" (as with anti-black racism); heretical nonbelievers; agents of destruction.*
The final part is that this guy walked into a synagogue on a Saturday morning during Yizkor (a memorial service to pray for deceased relatives) and opened fire with an AR-style rifle because they are easy to come by and cause unthinkable damage in mere moments. Gun violence is tied to white supremacy, in part because gun rights arose as one way of "controlling the means of violence and suppressing the voices of the disenfranchised." Indeed, the "well-armed militias" we are so fond of referencing in the Second Amendment were, before the outbreak of revolution, almost entirely organised out of fear and greed to suppress slave rebellions.
The gunmen himself, like the shooter in New Zealand before him, actually directly noted that he used a gun because they are the easiest way to cause harm and panic quickly--essentially, kill as many people as possible, as quickly as possible, while spreading the most amount of terror.
What do you get when you mix a mounting fear of outsiders overturning the established order (accelerationism), in this case Jews (anti-Semitism), with easy access to military-grade weapons? You get a white supremacist mass shooting in a synagogue. Or a mosque. Or a gurdwara. Or a hospital. Or a school.