So nobody is justifying it. Just explaining why some communities supported the IRA.
If you don't have equal rights, have no political representation, can't get a job because of your religion, or a social house, or vote and you can't even go to the police when you're the victim of a crime you might find yourself sympathising with the local paramilitary group who appear to offer solutions to some of those issues.
Remember that in these areas the IRA are people's neighbours, nephews, fathers etc. Not some big strange far away bogeyman.
And while some people left to avoid all this, most stayed and Sinn Féin is the biggest political party in the North now in terms of seats.
Edited to add that obviously poverty was rampant in many nationalist communities and so poverty and lack of options are also important reasons that people stayed.