With the Geth being a counterexample as well: they were peaceful until their creators realised the Geth had become fully self-aware and went full "kill it with fire" on them out of fear, resulting in a war that the Geth very quickly and decisively won.
And the evil Geth faction, the "heretics", were portrayed as brainwashed by the Reapers, ironically turning into what the Reapers were aiming to prevent.
Yeah I was thinking of the reapers, who [spoilers] did take over the entire galaxy, and decided to farm organic life from the shadows instead of extinguishing it (which works around the Fermi Paradox :) )
