Justice has been a very strong theme from the beginning of part 2, asa herself and other side characters have been fighting for justice, but its a twisted justice. Asa doing "what's right by her heart", Yuko & miri & haruka doing "what's for greater good" with different reasons.
It's to affirm their identity in one way or another. Yuko wants to be recognised, and learns from chainsaw man that people will love you if you do the right thing.
Miri wants to be his own person making his own decisions, and learns from chainsaw man to be "free" abiding any lie that's in the name of chainsaw man - because if he showed him how to be free, he must know what's right.
Haruka wants to devote his life to chainsaw man as a fan, he sees him as a hero, someone who gave meaning to his life. He wants to fight for a better world, because that's what heroes do.
But hold on! THERES MORE!
DO YOU SEE IT??? THE IRONY OF JUSTICE! JUSTICE ACTS AS VEIL, AS AN EXCUSE, AS A REASON TO LIVE AND A REASON FOR REVENGE!