Hero

Malala Yousafzai is an activist for female education. In her home of Swat Valley, the Taliban would prevent girls from attending school. Malala recognized that education is not a privilege, but a right to every human being. She began writing a blog under a pseudonym for BBC, but would eventually be in interviews and a documentary. Plato would say that she "left the cave". Her beliefs were dangerous though, leading to her being shot in the face on October 9, 2012, yet she still treks on. I could write pages about how much I admire Malala and that I wish I could be even half of the person she is. She's MY idea of a hero.
The official definition for hero courtesy of dictionary.com is "a person who, in the opinion of others, has heroic qualities or has performed a heroic act and is regarded as a model or ideal". So ultimately, it is objective to the person/society.  However, there is a common denominator between all heroes. Each stand up for what they believe in, as corny as that sounds.
With that said, Malala is a hero. Grendel and Beowulf are not heroes.
Beowulf is a very static and flat character. All we learn about him in Beowulf is that he is good at killing and he's quite barbaric. We don't know his inner feelings or aspirations. He isn't a hero if he doesn't particularly stand for anything. Grendel is a hypocrite. He scorns humans for rewriting history... But in writing his story, he is imitating the Shaper that he despises so much.