Question

why does Otto von Bismarck feel it was necessary to avoid doing great harm to Austria in its war against Prussia?

Answer 1

damaging Austria too greatly would cause its leaders to join with Prussia's other enemies

just incase you have other options

Answer 2

One of the main reasons why Otto von Bismarck felt it was necessary to avoid doing great harm to Austria in its war against Prussia is because he felt that Austrians were inherently Germans, and did not want to fight Germany's own people.