When might a historian's point of view become bias?

A historians point of view might become event bias if they come across information about it, that they don't like, their research maybe blurred.

Also historian is biased if it leads him to distort or ignore evidence that might contradict his views.

Explanation:

History often is portrayed as a single and united field.

However we need to recognize that inherently all Historians show some bias and that there is no singe History but (Stories to be told).

A historians background will often reflect on his portraying of facts:

Think of the vision of people on different sides of War, before and after the conflict. As such phenomena also influence his opinion , an effort should be done to portray facts but, this inherently makes the researcher focus on certain topics while discarding others , or giving less priority.

When complex facts are interconnected and some evidence is still being discovered a reinterpretation of past events can often take place.

Some prefer to recognize bias is unavoidable, as Historians interest directly influence their view of past events, the language they use, and their arbirtrary selection of evidence.