Mass Effect & Mass Effect 2
The harsh krogan homeworld conditioned the krogan psychology for toughness just as it did the body. Krogan have always had a tendency to be selfish, unsympathetic, and blunt. They respect strength and self-reliance and are neither surprised not offended by treachery. The weak and selfless do not live long. In their culture, 'looking out for number one' is simply a matter of course.
After their defeat in the Rebellions, the very concept of krogan leadership was discredited. Where a warlord could once command enough power to bring entire solar systems to heel and become Overlord, these days it is rare for a single leader to have more than 1000 warriors swear allegiance to him. Most krogan trust and serve no one but themselves.
This solitary attitude stems in part from a deep sense of fatalism and futility, a profound social effect of the genophage that caused krogan numbers to dwindle to a relative handful. Not only are they angry that the entire galaxy seems out to get them, the krogan are also generally pessimistic about their race's chances of survival. The surviving krogan see no point to building for the future; there will be no future. The krogan live with an attitude of 'kill, pillage, and be selfish, for tomorrow we die'.