We can quote many examples for such things which are helpful or harmful depending on point of view or situation or amount etc.
a) Bacteria : they can be helpful for fermentation process used in manufacturing of many useful things (like beverages) and can be harmful as they cause rotting of many useful things
b) Beaver : it is one of the largest rodent on earth. It is harmful as it causes cutting of trees. It lives upon them.
However, it a natural dam builder. It has a tremendous power to swim or float in water.
It can create canals naturally.
But for living purpose it feeds upon trees and cause destruction of trees and thus forest.
