I'm not quite equating what happened in Texas with war between sovereign states, but I do think there are some parallels in the mind of the terrorist, as well as in history and human behavior in general.
If you find yourself in a violent conflict with something intangible, like a government or a bureaucracy, how do you not see the people who are working for that government or bureaucracy as being part of the problem?
EDIT: I want to be very clear here. What happened to the employees at the IRS building is tragic, and they didn't have any influence on the IRS' decision-making process, or even on Stack's particular situation. Rather, what I'm trying to say is that as long as there is no way to peacefully resolve disputes in situations like this, to the satisfaction of both parties, then what happened is to be expected and will continue to happen. (Remember Tim McVeigh? He had some of the same complaints...)
So, what happened must prompt a quick response in the form of reforms of the IRS, specifically in dispute management and resolution, and I think that every article that handwaves Stack's terrorism as being the actions of "a wacko", "a nut", or "unjustified", weakens the case for those reforms.
It's only a matter of scale. Nations have gone to war for less righteous reasons; such as religion or taking stuff from weaker nations.
It is like any other big company. The call centre people are never responsible for the mess the company made to you. However, if you do not want to devote significant resources, you are pretty much screwed up and yell at the call centre people.
The difference is that, in the most cases, you can just forget about the corporation and live your life. There is no opt-out from the IRS, though.
I'm employed by a system which I believe is unjust. It extracts rents from skilled young people as the price of admission into the professional world, and extracts rents from scientists by all sorts of shady methods. I don't have any direct control over it.
But by working for this system, I am partly responsible for it. I can't stop the system, but I don't have to support it (and once my contract expires, I won't).
Any IRS employee who witness the injustice and continues to work for the IRS is partly responsible for it. They could leave, but choose not to. "I just work here" is not an excuse.