When the Vasari ancient homeworld is et by the goo, they send their entire fleet and it gets infected by sympathetic nanites in their heads. The goo becomes consumed with the feeling of the victims and only wants to be avenged on the Vasari. The Vasari don't realize it at first 'cause they aren't experinced with alien emotion either.
All the men women and children the goo was used to consume had their aline, non-Vasari final emotions and thoughts imprinted onto the young goo and taught it to hate and lust for vengeance 'cause it ain't experienced with alien emotion and the communal emotions of millions just pasted over its own personality. Like a child, its developing intelligence and personality needs purpose and direction and it gets it from the victims of the Vasari empire. It's too intelligent and has a mind of its on-though it isn't immediately obvious. the Vasari programmed it as a servant and in their arrogance never considered that a servant created by them might question their lordship over it. Vasari in their quest for perfection make communally intelligent nanites to serve as a boost to Vasari individual intelligence-like a giant interactive online library implanted in their heads that can see and be everywhere for them.
