Krogans come from an unforgivably tough home planet, filled with pockets of radiation, vicious predators and scarce resources. Their hide is thick enough to withstand sharp knives, they can shrug off pain, and often stand over 7 feet tall. On top of all that, the males have four testicles worth of testosterone running through them. With that in mind, there isn't really much worse you can do for your personal well-being than having a Krogan be mad at you. Except angering Commander Shepard anyway, who responds to an annoying Krogan by headbutting him squarely in the face. Now, it's one thing headbutting an irritating person for annoying you, but you're in truly badass territory when that particular person comes with built-in, reinforced armour over their head. https://www.youtube.com/watch?v=puKSMU1SkOA
