Heroes are self sacrificing, and use their abilities for the benefit and betterment of others. They do whats right no matter the cost to themselves. A hero is unwilling to risk others except him/herself or unless there is absolutely no other option.
Villains typically do what's best for themselves, and a few have a twisted view of things and do what they think, but is not necessarily the betterment of others. A villain also is willing to risk others to achieve whatever goals they have.
Like Doom or Apoc think that not only do they have the right to rule, but it's their destiny, and that by their hand they can better the world. They however are willing to kill others in the process, and bring down cities if they have to. Spiderman however is willing to give his life protecting others, and is unwilling to let others die if he can hep it. On a neutral side is Galactus who is essential to the MU, if he dies I believe everything dies with him. Galactus must survive as he says, however he kills many in the process, so he's neither villain as in a small way he's giving back more than he takes, but he's not a hero as he's willing to hurt others to survive.