I'm surprised noone here ever brought this up but in the leaked opening cutscene showed at NYCC it shows Nero asking Nico "How does it feel be rescuing the guy who killed your father?"
This line heavily alludes to Agnus being the bad because
A. In a leaked Nico's bio it heavily alluded to her father being a mad scientist
B. The first leaked DMC5 video has Nico in Nero's home asking for Order documents. I used to think she was from Fortuna but now with the fact her father abandoned her and her mom its a possbility that Nico either ran into Nero between the 7 year timeskip or searched for him so she could go to Fortuna and obtain her father's research which could be applied to the Devil Breakers or something else.
C. Nico's father is probably human...or human when he conceived Nico and Dante has only (as far as the main games are concerned) killed 2 humans Agnus and questionably Arius. While Lucia did finish off Arius its possible to say Dante was responsible for Arius's death too but since DMC2 is universally hated and Arius is the worst villain in the series I doubt Itsuno would reference DMC2 heavily outside Lucia AND dmc2 Dante's outfit (and that is probably for fan service).
D. Nico's model is biracial (with black being one of it) and thus this reflects in her character design (and more in earlier concept art) and Agnus is probably black (or of African descent) as well. Well his mocap and voice actor is TJ Storm....who is black.
E. They have said DMC5 is a direct sequel to DMC4 despite it taking place after DMC2. This means that story elements from DMC4 are probably going to play a big importance to the story. Such as how in the opening of DMC4 during Sanctus sermon Sanctus refers to a possibility in the future where the human and demon realm fusing and mankind being screwed is seemingly the plot of DMC5.....hmmm another potential theory there too.
EDIT
I trimmed down the original OP and thread to focus on this subject. I plan to elaborate more on this subject some more later and post the other topic in its own thread down the line.
EDIT 2
So I'm adding a few quick theories on this premise.
Theory A. Sanctus and the Order were preparing for Urizen, the demon tree, and demon insurrection
So like I said before Sanctus and the Order may have been aware of the demon insurrection based on his sermon at the beginning at DMC4. While yes the human realm and the demon realm merging have been possibilities in past games like DMC1, DMC2, and DMC3. DMC4 sets up that possibility again but doesn't do anything that alludes to it. This is brought up by Sanctus and never followed up in DMC4 since the Order's plans was to unleash demons onto the world (not merge the 2 realms) so they can use the Savior to wipe them out and convince all of mankind to unify under their religion. Agnus even implies this by saying "It takes a journey to hell for them to accept and praise their God." While the writers do make this stuff up as they go along they tend to base it off material and elements brought up before and DMC in a lot of materials are constantly talking about the 2 realms becoming one again as if its inevitability so the Order may have been preparing for that. It is actually unknown why after about 2,000 years they decided world domination was the best choice. Arius, Arkham, and Vergil never wanted world domination either power or immortality. You should take into account the Savior is very powerful and nearly indestructible and might've been more powerful if powered by Dante (who they initially wanted) instead of Nero. Dante actually couldn't beat it until Nero was removed and it was brought up in the DMC4 novel it was indestructible. Fortuna is arguably the most advanced civilization (or has the most advanced military) on the planet melding demonic magic and technology into one to create advanced automatons with AI that operate under their commands, demon enhanced soldiers, various advanced weaponry like the Red Queen and the other exceed weapons the Order uses, and all of Sanctus's demonic mechanizations. Being able to conquer the world and spread both their religion and technology would make the world more prepared for events like these. Now the question of whether they were aware of Urizen is possible since Agnus and the Order is being referenced again and V is very aware of Urizen. If Urizen is the big bad king of the underworld that is even above Mundus he should be known to those who look into this stuff like the Order. They should be Urizen or a demon king like him is building power and amassing resources to start a full scale invasion.
Theory B. Who is Agnus and why he left his family
I don't think Agnus is Nell's son unless Nell is now black too like Morrison or Nell's husband is black (I really want to make a tasteless interracial porn joke but must resist)...or both...unlikely. Nell's daughter is probably Nico's mom and Agnus married into the family or didn't. Nico could be a bastard child (must resist). Granted in the DMC1 novel it stated Nell had 1 child, a son who looked like Dante a bit but the DMC1 novel had been basically non canon since DMC3 and Morrison is now black so Nell now has a daughter instead of a son. Nell is brought up in DMC1 so at the very least a female gunsmith who made Dante's guns named Nell Goldstein exists in the DMC lore now with DMC5 we are getting a canon backstory for her and her family and in this new canon by Itsuno and Bingo she has a daughter now. Why a daughter and not Agnus because Nico is more likely to keep her mother's maiden name than her dead beat father who abandoned her last name. That is something that happens in real life where the child can adopt the mother's last name instead of their father's. I have a friend who has his mom's surname despite actually knowing his dad and occasionally seeing him. It is weird. Plus the Goldstein name is probably very famous and something Nico is very proud of. I do believe Agnus was a foreigner to Fortuna since he is the only person there who isn't European. It makes sense for a closed off European society that was formed before the slave trade and the era of ethnic expansions and exploration to be solely European and the only other "foreign" character at Fortuna is Trish posing at Gloria.....a dark skinned person. It seems the DMC writing staff thinks the best way write a foreign character for Fortuna is by making them not European/white. That and Agnus is rarely ever seen and is known to be a shut in that that was Nero's first time seeing him so many in Fortuna aren't aware what he looks like or that he exist. Another indicator of someone who doesn't fit in or belong.
Now who is he. Not sure maybe his backstory might be fleshed out in DMC5 but to me I think Agnus was some sort of regular regular scientist and engineer who was into this demon stuff and weaponry and had potential interest in merging the two fields. Probably married into the Goldstein family due to their acclaim and wanted to learn more about weaponry from master craftsmen. He may have discovered the Order upon his research or been approached by them or Sanctus due to his skill and knowledge and ultimately left his family behind to pursue his demon tech researach in Fortuna. Leaving behind his wife and probably infant daughter.
Ironic he gets killed by the same gun his wife's mother made. Almost like poetry.
