Seeing this prompt: [WP] As it turns out, humans are not the generic, good guy, center of the galaxy type species. Humans are a specialist species, and the rest of the galaxy only cares about one thing when it comes to humanity. Our explosives.

Got me thinking:

Humans are crazy. They fly around in ships filled with enough volatile chemicals that a single well placed shot would make the whole thing explode. On top of this, their greed made them willing to fight for anyone willing to pay. This completely changed inter-galactic warfare. Suddenly any faction could hire a bunch of insane maniacs and casualties would skyrocket. 

There was no winning a fight against humans. Even if the battle was a victory, the losses would be staggering. Everyone else tried to protect their soldiers by ensuring their ship crews could survive anything, resulting in most fights having very low casualties. Unless a soldier was unlucky enough to be hit directly by a shot, they could expect to survive even if the ship got disabled and filled with holes. Humans however, did not build their ships like this.

Human vessels would be packed with more firepower than a ship 5 times the size. Their weapons were not designed to disable the enemy ships for capture and harvest of resources, but instead they were designed for complete destruction.  I suppose when you're accustomed to fighting against other powder kegs, there's no point in aiming to disable. For defense, they mostly just had powerful engines (fueled by highly volatile chemicals of course) and tried to not get hit. Which of course, just encouraged them to make weapons that produced gigantic explosions to compensate for how hard it was to hit their targets.

The craziest thing of all: The ships I'm talking about aren't even their dedicated war ships. Those are just the mercs with fast response vessels. Their actual military ships? Well, just imagine a giant cannon with engines. That's basically it. The design is simple, cheap to build, and with commanders that care more about money than lives, they are all too willing to sacrifice these ships in deadly battles if anyone draws their ire.

Everyone thought these Humans would be weak. They were the newest member of the intergalactic community. Economically, they are weak. Yet, because of their insanity, no sane empire is willing to fight them.