Zak is pretty brutal.
Insane, quick to the point, and has no qualms whatsoever about killing anything.
But the big thing about him is that he Crits with a 1/4 probability, or something of the sort.
And when he Crits on a Basic Attack, he deals approx 150 damage, realistically, you're dead in one or two hits like that.
Oh, and he frigging makes Basic Attacks as a minor action.
So yeah, brutal.
But from what I've seen, Syn is about the same.
Insane, a murderer, no qualms about killing. In fact, quite like Zak it sounds like he enjoys it.
He's not so quick to Crit, but openning with his 7[W] attack is going to give you problems.
Mostly because that puts four shrouds on you, and his shrouds dont use up.
So that's an extra 4d6 + 12 damage each turn with a minimum of 3d6 + 9 even if he misses.
And then there are the souls that he stores.
A close match I would say, but I guess we'll see.