Just throwing my hat into the ring here, I think that the series itself shows us a fairly good metric for deciding whether a character is "good" or "evil." One of the repeating motifs is whether or not it is "right" to sacrifice people for an ambiguous greater good. Examples of this range from Tien's unit being sacrificed to things as large as the Diagram causing massive war and death for the possibility of saving Roshar. On a regular basis, the truly heroic moments of central characters are when they stand up to this mentality and refuse to capitulate. Kaladin doesn't kill Elhokar. Dalinar trades his Shardblade for the lives of all of Sadeas' bridgemen, and he and Kaladin have an exchange that encapsulates this in a microcosm:
Now, this isn't a clear dividing line, but there are people who, regardless of their intentions, fall on either side of these lines with their actions. I think someone can have the purest motives and be evil--and I believe that whether or not a character is evil can change, it's not a fixed quantity. But this equation is important, and in-context of the series itself is used as a dividing line between the characters who are moral--who are our heroes--and our antagonists. It seems fitting, for a world so defined by the presence or absence of Honor.
Interestingly, this distinction in the books places the Sons of Honor--at least, those we've seen--on the distinctly dishonorable side of the line, along with Sadeas, Taravangian, and Moash.