The fingers cutting thing is something that clearly drags the film into absurdist territory - I can see how people who wanted a more straightforward historical drama would find it jarring.

Rather than a allegory for a civil war, I interpreted it as a struggle of someone having an existential crisis about not wanting to just accept the familiar and mundane in life. But, rather than finding a new balance that allows you to explore new things, you fixate on burning the bridges to your old life. And ultimately all that does is harm yourself and the people around you. You see multiple examples of that theme in the film: With the sister taking the 'healthy' option of pursuing the new, rather than pushing away the old, whilst the 'idiot' kid loses all hope after his first half-hearted attempt at pursuing his 'dreams' fails. Farrell's character (can you tell that I don't remember any names) has no dreams at all, no capacity to deal with change - and unable to handle things when they inevitably do.