What do mafia hit men, Wild West gunslingers, Japanese samurai and the Knights of the Round Table have in common? They all work under a code of honor. Despite the danger, brutality and violent nature of their jobs, each of these examples has a strict code that helps them define what actions they are willing (and not willing) to do to get the job done.
Aside from alignment, most PCs don’t have any strict code that dictates their actions; although in previous editions of D&D the Paladin did have this restriction. Now it comes down to the player running the character. The only honor your character has is that which you instil in him. Honor, however, is certainly subjective. Two players who play their PC with an honor code are likely to have some differing opinions on what is allowed and what is not.
A common aspect in the code of honor is that women, children and innocent bystanders are usually exempt from any part of an ongoing conflict. If you’ve got a beef with a local merchant you won’t kidnap or harm his family as leverage. His business might be fair game, but his son or grandfather is not to be harmed. Assassins generally have a similar code; remember Leon’s motto in the movie the Professional: “No women, no kids.”