The most salient example of the bias I can think of comes from reading interviews/books about the people who worked in the extermination camps in the holocaust. In my personal opinion, all the evidence points to them being literally normal people, representative of the average police officer or civil service member pre-1931. Holocaust historians nevertheless typically try very hard to outline some way in which Franz Stangl and crew were specially selected for lack of empathy, instead of raising the more obvious hypothesis that the median person is just not that upset by murdering strangers in a mildly indirected way, because the wonderful-humans bias demands a different conclusion.
This goes double in general for the entire public conception of killing as the most evil-feeling thing that humans can do, contrasted with actual memoirs of soldiers and the like who typically state that they were surprised how little they cared compared to the time they lied to their grandmother or whatever.
The "people are altruistic" bias is so pernicious and widespread I've never actually seen it articulated in detail or argued for. Most seem to both greatly underestimate the size of this bias, and assume opinions either way are a form of mind-projection fallacy on the part of nice/evil people. In fact, it looks to me like this skew is the deeper origin of a lot of other biases, including the just-world fallacy, and the cause of a lot of default contentment with a lot of our institutions of science, government, etc. You could call it a meta-bias that causes the Hansonian stuff to go largely unnoticed.
I would be willing to pay someone to help draft a LessWrong post for me about this; I think it's important but my writing skills are lacking.
Was wondering how a criminal defense attorney could have ever believed that police shouldn't exist until I go to the end!
Eh... I think you and him are worried about different things.