I know next to nothing about this guy's portrayal as a character in novels, but my characters are in Ultramar and they may bump into him. So, one wonders:

What are his personality traits? How does he act? I've literally only seen satire of him, so, I'd appreciate any help in an accurate portrayal.

You can find a good lot of information about him here

If you watch there is a series on youtube called Emperor with a text-speech box or something along those lines, has a good representation of Mr. Sicarius imo.

I've seen it. Is that REALLY how he acts? Because I was dead sure that was a parody and if I do play him like that, my players, even as absolute fanatical loyalists in character, will want to kill him...

Isn't that a good thing?

Point. I may put him dangerously close to falling to chaos as well. A pridefully insane juggernaut as that portrayal is just asking for the chance to fall. Who knows, maybe the party will push him over the edge? Or even teach him humility? We'll see, when they get back from messing with tyranids!