Plus, achieving an individual lifespan of thousands of years might be a trivial feat for a civilized species, something we humans will do before the next century rolls.
Why come halfway across the galaxy to blow us up?
To ensure that we don't come and blow them
up! Sterilizing the galaxy of competition is a valid strategy of survival. Although one might invent more altruist motivations for berserkers, too - say, they might want to stop civilizations from becoming so advanced that they quicken the heat death of the universe by their technologies (Alastair Reynolds toyed with this). Or they might be on a terraforming spree, but over the aeons, something goes wrong and terraforming becomes extermination - a very realistic prospect considering the basic nature of terraforming.
And even with superior technology, how do you colonize and control seven billion people? You'd have to bring an army, and that'd have even higher energy demands.
Naah. Just enter orbit, spray everybody with an agent that will kill them (or worse) unless they behave and queue up for their antidote, and the biggest expense in all of this will be hiring the clerks who dish out the antidote pills. Then, at your liberty, introduce other useful chemicals into the daily antidote dose...