I know that this is not a true in Trinity Blood universe, and I am simply brainroted by my favorite childhood series, but I think that it would be really cool if Isaak was incarnation of evil.
Imagine - you do not know from where you came from. You do not know what exactly you are. Immortal in some way. Always alone in some way. You see people change under your hands; they become more cruel and violent as you speak to them. You are on this world because without you there wouldn't be goodness. You didn't choose this, this is simply how it all works, your life is a concept.
The character who is described in the series as 'Devil' is Dietrich. But he was raised by Isaak. Puppeteer seems to not understand exactly what is good, he knows that Panzermagier is evil.
We could say that Cain too is a devil in some way. But as we know, it was all different in the past.
Isaak seems to be manipulating everyone, planning his own world.
In conclusion - I may or may not think about this idea more deeply. Maybe I will made AU for this, but knowing myself, I won't do anything.