Some people using guns to defend themselves against who they believe are the harbingers of this authoritarian State is 2nd amendment working as intended. Not a "tragic but necessary sacrifice" as some will put school shootings, but actually what right to bear arms is supposed to be about.
And it's immaterial if you ultimately disagree to whether this administration is authoritarian, but these things will keep happening as long as enough people believe that to be the case. It's a feature, not a bug.
The Second Amendment fantasy is that you should own guns, so that you can kill people in the government and who are adjacent to the government. That means shooting real people with real bullets to kill them.
I think their reply is a criticism of the Second Amendment fantasy, rather than a remark that this is a worthwhile avenue for fighting fascism.
As others have pointed out, Charlie Kirk built a career on the Second Amendment fantasy, even explicitly delineating Democrats as targets he believes are acceptable to shoot and kill.
That said, I do disagree with the assumption that the shooter is necessarily opposed to the Trump administration or even to Charlie Kirk's rhetoric.