Also another thing that might seem insignificant was the color. All demons and abominations made from them are red when they glow, Justice is always blue. Does this mean there is a fundamental difference between demons and spirits aka they are completely different from each other or are they simply same species with different motives.
The writers are always very careful to avoid having anyone give a definite answer on whether they're fundamentally different species or not. In-universe, there are a lot of very opinionated people, but it's the Fade. How you see it probably depends on what you expect to see.
I was wondering if spirits reproduce by a kind of reincarnation, is the first identity completely erased...? The joke was just meant to illustrate that Justice might hate demons now, but if a death as a wisp erases all of the data of the previous self, how would he know he wasn't the thing he hates now?
Ultimately, the writers haven't given us enough information about fade creatures to understand their life cycle. But so far it's the idea that makes the most sense, given what we know, and it's too well-written and well-thought-out a world for them not to have given it some thought.
We do know that Justice doesn't know if there's a Maker - you can get that conversation. Since the part of the Chant that talks about spirits has them being given commands by the Maker, either Justice wasn't there, doesn't remember being there, or that part of the Chant is wrong (all possible).