Skoll Shorties

But how are Thrall and Rokhan evil?

Thrall in WoD blamed himself entirely for Garrosh and after WoD his blame for Garrosh manifested into his inablity to use the elements.

But his entire quest to make Garrosh pay for what he did is surfaced here and then afterwards that blame does sink in. The cinematic did do that but afterwards he does actually start to blame himself again.

You get more knowledge as to why the elements "left him" by reading the Lore Books from the Shaman Order Hall on the Doomhammer its all about Thrall's guilt over killing Garrosh. Thrall was responsible but countlessly tried to repair what damage he did. He's by no means evil.