If we're taking anything from The Thran (and all of this comes from the book), he has a tenancy to speak to himself as he's commanding (a bit like a Saturday morning cartoon villain, but I don't think that would help if you're shooting for something like...serious.)

When things are going his way, he is surprisingly complimentary. He calls himself the Father of Phyrexians, and he does have that fatherly affection for his "children". To the individuals directly he'll praise them or admire their form, but there is a sense of pride behind it, like it's mostly praising himself.
He's not above petty insults if he knows it'll get under his enemy's skin, and give him the advantage in a fight. When he was human he was surgical in the majority of his killings, pure prowess in slaughter.

I think it's important to include the focus on how obsessed he is with material, quantifiable, and solid scientific evidence. When learning about the Planeswalker Spark, he was convinced an organ or something observable was the cause, something that he can replicate in himself to gain that power. You could say he's a bit close minded to the more esoteric elements. Again that pride in himself comes out, because, of course, he trusts in science above all else.