Honestly, the dragon is just extrapolating what he knows of both Medivh and Velen. He's talked to neither individual; Medivh is dead, and Wrathion is uncertain as to whether the Draenei's leader would spare him the time. (He knows Velen not at all. Likely the Prophet would have some interesting words for the Black Prince, especially considering the dragon's interest in Velen's former protege.)

Being informed that Bruno 'only saw bad things' perplexes him a bit, and he hunkers down to take a seat on his haunches, lifting a handlike foretalon to gesture. "So...you see bad futures, and yet instead of taking your information to account, and doing what they could to ameliorate it...they feared you and ignored your advice? And blamed you for it?"

There's nothing but irritation in his voice when he grumbles. "Yes, that does sound like humans as a group. Individuals are fine, but get a number of them together and they're paranoid and fear what they don't understand..."

Part of that is because he ought to know, but again, it isn't as if Wrathion himself is not paranoid...