There are several things that distinguish heroes from monsters, especially in classical literature. Heroes and monsters are very highly contrasted through the dichotomy of good and evil. Heroes are often wholly good; monsters are often wholly evil. Being "good" entails protecting the wellbeing of others, and being "evil" means causing destruction and harm to others. They are also very starkly different in their physical appearance and personal attributes. Heroes, especially Beowulf, are bold, strong, physically fit, usually handsome, brave, wise, etc. Monsters are usually grotesque, horrifying, oversized, etc. Hope this helps.
