If you're treating cops and guards at an immigration detention facility as "non-civilians" and "combatants", then this is a good sign that the thing you're doing is engaging in some kind of ideological rebellion against the state. Which is fair to call terrorism, unless you're in favor of it, in which case you'd call it freedom fighting or rebellion but with positive valence.
People who are above the law and shoot at other people with impunity are combatants. What the fuck are we even talking about? Ideological rebellion against the state alone does not terrorism make.