Why do we assume that the dangerous aliens developed the tech?
What if instead the galaxy is full of intelligent species and an advanced ethical species spread its technology to less ethical intelligent species without consideration of what would be done with that tech? What if that less ethical or more primitive species simply stole the tech?
Think of our own history where advanced European tech such as guns was rapidly absorbed by hunter-gatherer tribes. They couldn't make them but they could use them.
Imagine what we might do if tomorrow we obtained a fleet of FTL ships. Would we act ethically or go hunting for other planets to colonise? With certainty some of us would go marauding.