BTW the original injunction of "Dan lekaf zchus" - "benefit of doubt" in approximate translation - comes from Ethics of the Fathers, and I believe one of the major commenters (R.Yonah IIRC) suggested, essentially, giving heavy weight to the prior: if the person is generally good you should try to explain an apparently bad act, and vice versa! you should explain an apparently good act of a bad person UNfavorably. Pretty sane thought.

I'm not sure. One can see how this goes wrong in Talmudic contexts. For example, there are a lot midrashim that explain away apparently good behavior by Esau and Ishmael, and there are a lot of midrashim that explain away or try to justify apparently bad or deceptive behavior by Jacob. Yet, a simple reading of the Biblical text shows that what is actually happening is that these just aren't 1 dimensional characters. So this general tendency can be actively distorting.

Edit: For others reading, midrashim are a classical Jewish set of stories generally told in an interconnected fashion to fill in apparent gaps in the Biblical stories.