Another factor is that Paul has forgotten what his outlook was like before the war -- he can approximate it in his memory but he can't actually "remember" it enough to put on the facade. The trauma of war has damaged his ability to simply sit back and experience normal life; the superficial people around him seem facile and shallow, with no real idea of his experience and how it has affected him.
The injustice surely relates to the way in which Paul hears others talk about the war and what it must be like to fight in the war. The point is that these people have no understanding of the reality that Paul has just experienced, and therefore talk about their romantic notions of war rather than the truth. Paul is disconnected from his "normal" life because of his experiences.
One moment of a kind of injustice occurs when a civilian and Paul discuss the war when Paul is one leave. The civilian fancies himself a military expert who can see the big picture, whereas Paul -- who has actually served at the front -- supposedly has only a very limited idea of how the war should be fought:
"Now, shove ahead a bit out there with your everlasting trench warfare--Smash through the johnnies and then there will be peace."
I reply that in our opinion a break-through may not be possible. The enemy may have too many reserves. Besides, the war may be rather different from what people think.
He dismisses the idea loftily and informs me I know nothing about it. "The details, yes," says he, "but this relates to the whole. And of that you are not able to judge. You see only your little sector and so cannot have any general survey. You do your duty, you risk your lives, that deserves the highest honour--every man of you ought to have the Iron Cross--but first of all the enemy line must be broken through in Flanders and then rolled up from the top."
Paul's opinions are not taken seriously, even though he obviously knows more about the war than the man who lectures to him about it. No wonder that Paul finds his visit home alienating and dispiriting!
Paul returns to his home and I guess we could say it's so far removed from the war that it may seem a surreal experience to Paul. He has been at the front, "up close and personal," and there is a stark disconnect between those still wrapped in the innocence of not truly knowing the stark realities of war and those who can think of little else. There may be shortages of food and such, but all these people have to judge the war on are stories. Paul does all he can to keep the real truth from his mother because she is sick.
Paul sees familiar sights, but things don't feel familiar to him. This is because while the town is unchanged, Paul is not. He may be able to recall memories of his youth, but that person is gone forever, lost in the ferocity and carnage of war. Even in his house, there is that separation, and Paul notes...
...but I am not myself here. There is a distance, a veil between us.
If there is any injustice, perhaps Paul experiences it when he meets an army major in the street who dresses him down for "behavior unbecoming..." This seems unjust after what Paul (and others like him) have gone through.
And while Paul changes into his old civilian clothes, they don't fit; symbolically, this represents the many changes Paul has gone through, supporting again that sense of disconnect or alienation.
To me, the alienation that Paul feels isn't so much because of any injustices but because of the fact that everything at home seems so normal. He realizes that he does not feel like everyone else. He realizes that there is this big gulf between him and them in terms of their experiences. Therefore, he just can't feel any connection to normal society.