Can't speak for everyone; but that would violate essential freedoms. We're talking the wild fringes here when it comes to cold blooded murderers, it is better to tolerate a few leftist crazies.
But it is still a worry; right wing politicians get targeted and there is a lot of incitement on the left encouraging it.
No, they're worried about crazies with guns too. Eg: https://en.wikipedia.org/wiki/Congressional_baseball_shootin... was a left wing activist targeting Republicans.