In the old canon Thrawn enslaved nations (like the Noghri) used torture, mass murder, terror and subterfuge to reach his goals.
He supposedly just wanted to unite the galaxy to be able to fight the Juuzhan Vong but you'd think that just saying "Dudes! There's these alien warmonger horde and it's coming right for us! We gotta fight together!" (or some more eloquent way to phrase that) would have also been effective AND it would not have caused millions of deaths in a unnecessary war that simply made the galaxy weaker.
...But that's just me and I don't know what the current canon on him is because Season 2 of the Rebels is still unavailable on Netflix...