And from what we hear in the audio of him in the final encounter and how he regards Thrall, its at its most obvious, as he talks down to Thrall in the same way he talks down to all others, seeing everyone else as weak.
Its remarkable really, Grom, even at his worst, had more guile and noble qualities then Garrosh. And yet Grom was corrupted by demons.
Garrosh has been corrupted by nothing but lust for power, and yet he comes off far worse then his own dad.
those are not qualities to be proud of. Think about it, Grom did this:
After he drank mannaroths blood a second time, slaughtering dozens of elves after doing so.
Where as Garrosh, who has nothing to owe any of his rage and bloodlust to, has premeditated thoughts of rivers running with blood, standing on a mountain of skulls and ruling the world.
Yes, Garrosh is worse then Grom.