By the time the GFA came around most if not all the grievances of the Nationalist had been addressed.
No they had not. As you said yourself, NI was under direct-rule from London.
i.e.- citizens of NI had next to zero say in how their province was to be governed.
Politicians without a single vote would decide.
Mostly Tory, and Unionist leaning. So when RUC and B-Specials violence erupted on the civil rights movement at the behest of the Unionist Party ("its a front for the IRA") the British response was not to chastise those in authority but to re-enforce the Unionist domination by bringing in the British Army and direct-rule. Then internment, curfews, torture programs, slaughter in Ballymurphy, Derry, criminalistion, shoot-to-kill, collusion, cover-ups, etc, against the Nationalist/Republican/Catholic community.
And lets face it, the RUC enjoyed next to zero confidence from the Nationalist community until eventually Unionists and the British government faced the reality of police reform through all-party negotiations.