My history teacher in Wales said “Without opinion the sources are boring – bland” (OWTTE).
In IB History we don’t really say “Bias” anymore, because an opinionated source is just as useful as a relatively neutral source. I say “relatively neutral” because there is no such thing as an opinion-less source. We use the model OPVL (Origin Purpose Value Limitations) to use the Origin and Purpose to talk about the Values and Limitations of a source. So we’d say that this guy is an American right-wing historian and is trying to villainise the left wing Russians or whatever. But the problem comes if it’s say a right-wing holocaust-denier who is trying to talk about Germany. The historian will factor out evidence that is accepted in order to push a revised view.
And so it seems to me that the difference between bias and opinion is the intent and the manner. When the author goes beyond the meanings to select harsher words, there is bias. I could say “Stalin was a mass-murdering monster obsessed with consolidation of his power” or I could say “Stalin was responsible for the deaths of millions of Russians, Slavs and Asians in his consolidation of power”. The difference is one appeals to emotion and attempts to slander the man. The other deals with facts and doesn’t dupe the less careful reader into a polarized viewpoint.
I am likely missing the point though.
(This is where the teachers nod)