> Do you treat other mass shooters and terrorists the same?
Of course not. But there is, more often than not, a common background between all these cases. Recognize and fix the system and prevent more suffering. Shootings are the ultimate outlet, so if you fix the system you automatically prevent loads of more suffering.
> By this logic, if employer in right to work state fires someone and that someone shoots the workplace, employer caused the shooting.
Yes (except for the rare case where an immediate firing is warranted, such as stealing from the employer, sexual harrassment or similar).
There is no obligation to exercise the "fired on the spot". You can as a boss always behave like a decent human being and show your employees at least a fucking basic level of respect. Or you can decide "uh we dont need XYZ any more, even if he's still productive, has worked years of overtime, shown dedication and has to support a family but if I fire him I can make 10k personal bonus that year"... but then I would not be surprised if you'd end up at the wrong end of a baseball bat or worse.
By the way I would advise you to read up on how most of the workers' rights we know today in civilized areas of the planet (right to unionize, strike, illegality of "at will employment", social security systems) have been achieved: through shitloads of blood. If your state still has legal "at will employment", the state is responsible for any avoidable suffering caused by this.