I think that some acts of evil and heroism are probably situation-ally driven, but that would probably explain more of "bad" and "good" than what we consider "evil" or "heroic". Those situations usually conote extremes in behavior and, from what i've seen, come from tempering yourself and your attitudes over time. For example, I doubt that someone would go from being an honest and actually caring person to being a sadistic child torturing rapist and murderer based on a situation. Equally, I doubt that a person who regularly skirts responsibilities and burns other people when it is convenient for them would put their life on the line to save someone else.

As with most things, extremes take practice.