I haven't read the comics myself, but it seems that the king is playing the opposite side of things then Thrall had. Thrall saw the generosity of humanity while he did openly despise their cruelty. Problem with the Alliance, and Human's in general is that they're xenophobic, in the case where you look more like a monster they will generalize that their nothing like them. Its a branding thing really, and a very real thing in respect to our own cultures.

The King time in the arena just assured him how savage the Horde really are. To him, keeping his hand at bay has been difficult but he also has been through a lot, and trust probably isn't high on his list.

Varian Wrynn's reasoning makes a good deal more sense when you consider his home, his father, and his mentor were all taken from him by orcs, even before his time in Durotar. He's not truly a warmonger, but he has come to the belief that "Live and let live" isn't going to work.

He may not be right, but his reasoning feels very natural. I really like the character and wish a mediocre comic book wasn't the way he was introduced.