There's power in stories

Justice: This elf should be brought to justice. Why should we assist her at all?

Justice: You cannot mean to have this murderer join our ranks! She should not be allowed to escape justice so easily!

Justice: Sigrun may agree, but I do not. This monster murdered countless Grey Wardens. They must be avenged!

Justice: So you are a thief. Nathaniel: Me? Not by profession, my friend. Justice: And yet you were imprisoned for attempted theft. Nathaniel: Of things that belonged to my family, and which were taken from us unjustly. Justice: A likely tale.

criminals are criminals. justice does not care about the justification or rationale behind the deed. he cares that the deed affected someone who did not deserve to be affected, and he wants that wrong to be righted. it doesn't matter that there is a long history of violence between humans and elves. the humans velanna killed were innocent, therefore they deserved justice. it doesn't matter that she's desperately trying to find her missing sister, because she is a murderer. it doesn't matter if you were robbing back from someone who had already stolen from you, because you are still robbing someone. it doesn't matter if the deaths of a few dozen people will result in saving countless more lives in the long term, because you are still murdering a few dozen people. a spirit of justice would not compromise its own morals to justify a desired end; to do so would make the concept of justice as a concrete ideal obsolete.

so yeah. something's changed. justice is no longer driven by the rigid set of morals that previously guided his actions and thought process. the justice that still lived in the fade would almost definitely look at the justice that blew up the chantry and be disgusted. i think at the most barebones definition, that does count as "corruption".

buuuuut. i don't think anders was right about it meaning justice is now a demon. and i definitely don't think he was right about it being caused exclusively by their joining. i think this has been a gradual change that began the second he was ripped out of his realm. justice's character arc in awakening is essentially one long, beautiful, terrifying existential crisis. the mortal world is so beautiful. mortals are so fascinating. he wants to experience everything. what is a sunset? what does love feel like? why does the concept of death scare him so much when it was never a fear he possessed before? these aren't inherently negative concepts, but they are leading to uncomfortable feelings.

and that's exactly his fear. he thinks these feelings are a corruption and that he's becoming the very thing he despises the most. he's jealous of kristoff for having a wife who loves him so much that it persists beyond his death. but envy is a sin. it's inherently based on selfish desire, and selfishness is the very antithesis of justice.

and that's why he approaches the warden commander:

Justice: I continue to envy their love. But envy is what a demon feels, a desire for something it cannot have. Warden: There's no harm in wanting something beautiful. Justice: I... think I understand. Justice: You co-exist with both great darkness and great beauty. It must be confusing.