That dream is pretty specific. Unless I have misunderstood what Jim wrote, the curse was meant to kill Eb's bloodline. Not all wizards, everywhere. If the power levels were this high it was like hunting mice with an elephant gun. That makes my WAG generator go off the rails.
However, the reversal of that curse did kill off another bloodline, a race of vampires that were dispersed world wide.. So Harry could have heard their cries as they died. You leave out the fact that the originator of the spell was the Red King, that power level was his, magnified by the gods of C.I., not Harry. However it did trigger a horrible vision.. What you quote is from Blood Rites, from an angry injured young wizard who has discovered the murderer of his mother.. That is a nasty quote, he felt that, but it was impotent, he didn't kill a mouse with an elephant gun, his efforts had no effect on Lord Raith..
I think being a star born puts one on a razor's edge, like an archangel or a god, awesome power, that can create as well as destroy. Harry has felt and wielded the power of both hell fire and soul fire, he rejected one, he is still exploring what the other is and it's uses.