Having studied this topic, I know quite a deal but will summarise is the most non biased way I can.
Working class Catholics demanded better rights, often employers descriminated against them (Such as H&W ship yard in Belfast). These Catholics wanted reform and underwent a series of marches to get this reform.
Working class Protestants and Unionists often argued that they had it no better than the Catholics and they were simply arguing for the sake of it.
Follow of series of high intensity marches by the nationalist/catholic community. I.e Belfast to Derry, and Coalisland to Dungannon. Tensions ran high in Northern Ireland, with a few attacks being recorded.
At this point in time, 60's onwards, the IRA, for the nationalist community. Stood for - I Ran Away. So the provisional IRA was founded, as a means to protect catholics/nationalists as it though the RUC were not doing their duty.
After a series of attacks by both sides of the community, but climaxing in 'the battle of the bogside' it was argued that the RUC were no longer in control of the situation and The British Army were drafted in. Nationalists greeted this very favourably as they though the British Army would be much fairer than the RUC.
However it was soon evident that instead of taking orders from London, the British Army were taking their orders from Stormont (The prodominently unionist assembly). This was highlighted during the falls road curfew in which many houses were ransacked by the British Army in a bid to find weapons. Membership of the PIRA soared.
In an effort to protect the protestant community from what they saw as more and more nationalist attacks many unionist paramilitaries were set up. Most notably the UDA and UVF. Fast forward and there are killings on both sides, often motivated by revenge.
The British Army introuduces Internment as most paramilitary members could not be prosecuted due to witness intimidation and the refusal of IRA members to recognise the British court. All members interned in the first 6 months are Catholics causing huge upsoars in the nationalist community. As more civilians are killed membership for both sides increases, as cousins, fathers and sons, outraged by civilian killings join up to defend their families.
This continues throughout, but the 70's are by far the worst years. Killings fall and the good friday agreement brings and unsettled 'peace' to Northern Ireland.
Conclusion, both paramilitaries committed horrible attrocities. No doubt. But on both sides many fought simply because they thought if they didn't they would be killed. Many fought to protect what they believed in. And the IRA cannot be solely blamed. We cannot forget the attroctities of the Shankill Butchers for example.
If anyone is still a bit hazy, which you will be. I'll write more.