I think that whistleblowers are heroes as they expose something that is going wrong and try to make it better. All too often governments or major businesses try and get away with unethical or flat out illegal activities that may hurt others, and thanks to whistleblowers these activities are exposed and come to an end.
Whistleblowers alert people to dangerous situations that they might not otherwise be aware of. They are the people that prevent tragedies, and are a vital part of society. Although some might find them annoying, without them more potentially hazardous situations would be abound. They protect the safety of people despite the possibility of being labeled a snitch, and are, thus, heroes.