What if Dillon Roof has a switch we can find and identify and create a vaccination so that the Adam Lanzas, Gerald Loughners, the Columbine kids, the Dylan Roofs of the future are immunized against that? What if killing him is killing off a reasoned chance at gaining that ability?
So, you're suggesting Roof become a lab rat?
"Never let the old man in." - Clint Eastwood quoting an old friend