We know that the heroes Sam and Dean are in fact criminals. They have no income, steal money, cars, credit cards, impersonate FBI agents and so on. This all can be ignored (by most people's moral standards, I think) to serve a greater good.
But where it becomes a little more brisant in my opinion is when it comes to killing humans. They regularly kill demons or angels together with their hosts, despite (at least in the case of demons) they have the alternative to exorcise the demon and save the human host. While there are cases where the human host can not be saved because their body is injured deadly and only kept alive by the demon, or perhaps the host is possessed for such a long time that nothing of the hosts soul is left, there are many occasions where Sam and Dean had the option to exorcise but choose to kill instead without a second thought. This could be justified with the assumption that a demon that is exorcised will eventually return and kill more people (so trading one life to save many) but this is harder to accept by most people's standards, I think.
An then there are those cases were someone they know and love is possessed. In this case they go to great lengths not to kill the host and to save them. This implies that in their moral conception the life of some people is worth more than others (and justifies that the exorcised demon will kill many people in the future).
This and other moral dilemmas in the show (like Castiel destroying the life of his host and his hosts family) are sometimes discussed by the characters or even plot points (but only once in a while), so we can assume that the writers are aware of this.
So my question is:
Out of Universe: What is the moral message the writers want to deliver to the audience? That the Winchesters are cold blooded killers? Or that all the kills of innocent people are justified somehow? Are there any interviews revealing what the writers intend the moral state of Dean and Sam is and how they manage to justify their kills?