I wouldn't say that ending is terrible (except from Sandra's perspective, heh).
Setting & Plot
If priests known religion is real, they could help Sandra out. Alternatively, Sandra can summon another fallen angel to prove it (if only she'd asked Billy how he met Ereiel!), unless there are more requirements than the blood of an innocent and a crude circle.
Demons can either manifest writing or perfectly copy handwriting, and if he manifested the journal, there'll be no receipt for it. More importantly, Sandra was pretty beat up, how did the examiners consolidate that with the journal? Did the demon clean up before leaving? I know murders can get investigated pretty thoroughly, but I'm sure it depends on circumstances.
With hell being literal alongside other mythologies, it does make me wonder if it is a belief or geography based thing. Pa certainly failed to exemplify Christian values, but that'd hardly be a mark against realism.
Ending
I'll let Mizal cover the big picture, and I'll say that there is no obligation to have a 'good' ending, and 'bad' endings can work.
The issue is that 'bad' endings often end up being edgy (and here I use edgy to mean shocking but devoid of meaning). You certainly have shocking elements here, but are they devoid of meaning? Given the demon's speech, there was at least an attempt at meaning, which I appreciate.
Pivoting to a 'bad' ending might mean you don't set it up sufficiently in the earlier parts tho.
Anyway, while the demon could've been lying, given this is the ending, it does seem him talking about 'the universe [having] certain rules' was exposition & the story moral.
Redemption for past failure being impossible seems like an odd moral to me, considering the role of redemption in Christianity (but being a universal rule stops me from questioning this as a world-building issue). I suppose the contrast is neat tho (but it doesn't feel explored).
However, while the moral itself doesn't sit right with me due to how absolute it is, if we ignore my personal disagreement with it, the real issue is that it renders the prior story a bit moot.
The cycle of abuse is not perpetuated because it shaped/traumatised Billy and his relationships for life, it is perpetuated because Billy summoned a Fallen Angel who allowed his Pa to meet a demon (by killing him), who then abused Sandra. If one considers oblivion better than eternal torture, Pa got the good ending by having his soul eaten. This ends up contradicting being 'marked for all eternity' due to failure, since Pa failed at both being a good Pa and at being a monster (he failed to break Billy), so he should've also been stuck in some 'cycle'. I feel this undermines the moral.
The Angel mentioned in the titles is also reduced to a plot point, Billy could've found a gun manifested by Norse Fairies and shot Pa, I don't think the ending would've been different. Maybe I just need to reread the earlier parts to see how they fit together (not like I'm the best at spotting thematic through lines anyway).
Setting up the ultimate inevitably of failure early could help it all read more like a classic Greek tragedy, which certainly didn't have 'good' endings.
The part at the end with the therapist lamenting that demons aren't real seems to point at Sandra's situation being tragic, but it rings hollow because demons are real in the setting. If it is commentary on the woes of irl humanity, it falls flat for me since I'm still focused on how a fictional demon ruined the life of a fictional protagonist. It might be a bit too short to offer deep insight, but maybe it had a better effect on others?
Anyway, the actual execution does still seem good to me, since the scenes are strong, so as I said at the start, it isn't a terrible ending. The tone shift into very dark maybe feels a little sudden, but I don't think it is inappropriate given the circumstances (i.e. literal demon). It isn't exactly out of left field either, considering Billy's past.
Maybe it just feels like it could've been better? Maybe I feel it cheapens the previous instalments? I'm not sure. Maybe I was just invested in the characters and am sad to see the story end like this.
Conclusion
The pessimistic moral seems like a departure from the previous instalments, perhaps it needed stronger foundations to feel more concrete, but as it is, it ends up feeling like the real moral is 'shit happens' (which is rarely satisfying/cathartic). However, the ending is only terrible for five of the characters, and is itself alright.
So, while it is not what I was expecting, it still ends the story strongly (even if I still question that moral).
Looking forward to whatever future writing endeavours you undertake, as I'm sure they'll also be engaging.
P.S. Like an absolute idiot, I ended up losing my initial write-up, so forgive me if some of those frustrations ended up coming through in my tone, lol. I liked my first version of the 'setting & plot' better. Anyway, good work finishing this, it wasn't a small project. Feel free to question any of my points here, it is easier to do when the work is finished (since no new information will be revealed), so if I misunderstood anything it is easy to correct me.