Alan Dean Foster wrote a trilogy based on this premise of humans being the ultimate insane warrior race. The Damned was about two collectives, one a mind controlling bunch of aliens called the Amplitur who absorbed other cultures as semi-mind controlled communists, the other the allied races of the Weave. The problem was no race was actually good at war.
And then they found humanity … and humans were insane. And they fought well … so well they scared the hell out of everyone … and were naturally immune to the mind control powers of the Amplitur.
This only made the weave even more nervous about their new found allies ….