Why be good?

[Angrboda]

This.

We evolved to be a social species and certain behaviors/values promote our success as a social species.  So part of our evolving social behaviors involved developing a meta-cognitive faculty we call moral judgement.  We intuitively value good behavior, and are repulsed by impulses to engage in bad behavior.  It's a subconsciously driven system of influencing what our consciousness wants so as to promote behavior that benefits a social animal.  In short, evolution built us to desire the good.  (This basic desire is also shaped and reinforced by childhood training, along with an inbuilt desire to belong.)

jorm-

Stalin killed 30 million well-evolved men, women and children who were weaker than he was. Their evolution did not help them. 

And from HIS perspective, why be good when being bad reaped such benefits?

Because on the whole, the species condemns such actions as not being acceptable. Only 50% of Russians polled had a generally positive view of Stalin. And that's only the picture inside Russia. Here in the west, the oppressive regimes of the Soviet Union were largely condemned as being contrary to the interests of the people. Evolved traits present a spectrum of behaviors from predatory wolves to those who play the social game to sheep that shy away from participation. The temporary success of an outlier is not as important as the behavior of the center, because that's where the real power lies. There is a truism that it isn't small business which creates jobs, it's the customers that create the jobs by demanding specific goods. In a similar way, the real norms of society are set by the majority in the center, not the outliers. A few million Stalins, all clawing to reach the top is a recipe for disaster of the species, and the species as a whole pushes back. Hitler's vainglorious attempt to dominate the western world was opposed by his peaceful fellow countries, and the push back nearly destroyed Germany as a nation. The power is in the center.