my charcter joined dawnguard and became a vampire lord way later( i was bored and curious). so im riding on arvak and all i here is Death to the dawnguard! i dispatch the assassin easily and read her letter i then go to the ruins she came from and killed everyone with a master destruction spell. But i have one question if i killed harkon why would you even try to kill me and they must have known i had auriels bow.....

Revenge?

Vampires in Skyrim are dumber than rocks. Just witness all the suicidal attacks on towns.