In which case you'd need a strong internal investigatory services in order to root these plots out before they happen by following up on leads and tips.
Well... not to get political, but I think we're hollowing that out too?
Who will necessarily be so strong they'll be capable of pulling such things off to serve their own ends.
It's an intractable problem all the way down.
My point isn't that outlawing guns would stop every possible scenario. Rather it would make killings of all kinds far less likely, which is a win for everyone--even hate-spewing pundits.