I think the real distinction is that terrorists are non-state actors who use violence (bad). States are legally allowed to use violence (necessary).
But the common parlance of terrorist usually means "US designated terrorist" which really means "group we refuse to bargain with".
The Taliban were a terrorist group, now they are a government. The Iraqi army was a legitimate army, then many became ISIS, a terrorist group.
You can be one thing, then change to another thing. It doesn't rewrite history.
A terrorist intentionally targets civilians, and non-terrorist does not. That's it.
A non-terrorist might kill civilians, but not intentionally.
When a leader targets their own people, we don't call them terrorists. My point is that the distinction comes down to whether the state is recognized as legitimate or not, and it's really just arbitrary.
Of course we do, look at what we said about Assad in Syria.
It's not arbitrary at all, people just say that to try to engage in moral relativism. The line as not as fuzzy as you are making it out to be.
The US doesn't target civilians- that's a war crime.
That is what I mean by the state monopoly on violence. You can't legally beat your neighbor. A cop can. You can't shoot protestors, a national guardsmen may be able to.
The state devises a legal framework to justify its violence. Operating within this framework is designated as "peacekeeping action" or "law enforcement". Operating outside of it becomes "violence". Even though the actions are the exact same. It's all just kind of arbitrary is my point.
My point is you can either be realistic and save yourself a lot of headache, or you can be idealistic and waste a lot of time and money because of your own hubris.