Die honorably in battle and you will go to Valhalla!
No, really, a hero IMHO is basically someone who risks his safety or comfort to help others. A soldier who fights for his country in a "just" war is risking his safety to help his family and countrymen. A firefighter risks his safety to aid people in danger. A political activist may in some cases risk his comfort (or even safety) in an effort to help people be aware of a danger or threat to them, or to help someone unjustly treated. A teacher foregoes his comfort when he deals with a "difficult" or abusive student, in an effort to reach out to him with a good example.
Such people are all "heroes".