Okay... Hollywood Lies are defined, and we're ready (God willing) for part II of the "How on earth does a Christian form his/her conscience properly?"
Start with the most basic of moral principles:
Moral Law #1: The end (i.e. goal) of an action never justifies evil means.
In practical terms, this means that no one is ever morally allowed to intend an evil act (or outcome), no matter what the potential "payoff" might be. If I could save a billion people from agonizing deaths by the murder of a single innocent child, it would be morally wrong for me to do that. If I could save a billion people from agonizing deaths by the euthanasia of an Alzheimer's patient who was to die in 6 months anyway... it would be morally wrong for me to do that. NO ONE can intend an evil act, even if the potential "payoff" is mind-numbingly "good". Evil is evil, and it must not be chosen. To say otherwise is to say to God, "You designed and You run the universe so badly that I need to break Your Laws in order to do Your Will (i.e. what is good)!" A few moments' reflection will show how absurd that idea is.
Note: make every effort to keep in mind the proper place of emotions; they are good things, created by God, but they were never designed to replace your intellect (the power by which you "know" things), or your free will (the power by which you "choose" things). Just as Faith, Hope and Charity ultimately reside in the Will, so do sins reside in the will. For example: "feeling" angry is not a sin; but the free *choice* to *nurture* that anger (i.e. hold a grudge, or inflame your anger into unjust wrath) is a sin.
I mention this, because one of the most common (and fierce) objections to these principles is the cry, "I couldn't possibly bear to do (or not to do) [x, y, or z]!" That's your heart, taking... you know, the thing that's "deceitful above all things"? Don't trust it to run your life. The heart needs to be directed by the will, which needs to be informed by the intellect; that may seem "cold and unfeeling" (it isn't!), but that's how not to fall into moral evil. Too many "heart-led" people have "followed their foolish hearts" and killed their unborn children, or murdered their disabled and/or elderly relatives/friends, or violated and betrayed their marriages in order to "follow their hearts" to "new soul-mates". In short: the heart is good; but it's not a good driver.
Moral Law #2: If an action has multiple effects--both good and evil--the evil effects are not sinful to the extent that they are not willed.
This is often called the "principle of double-effect". If you intend an action to cause [good outcome x], but it also causes [evil outcome y], the action is not necessarily sinful for you, provided that it satisfies four basic conditions:
1) the intended action is good, or at least morally neutral
2) the evil effect is not intended
3) the good effect is not caused by the evil effect
4) there is sufficiently grave reason for permitting the evil effect
(See HERE for details, on this.)
So... let's try a few examples, to flesh out this principle. (In the below, when I say "lawful", I mean morally lawful; civil law is irrelevant, to the extent that it violates the moral law, anyway.)
Situation #1: Is it lawful to kill an embryonic human baby (for the purpose of "harvesting" his/her embryonic stem cells), so that potential (or even guaranteed) cures for others may result--even to the extent of saving lives?
Answer: NO. The saving of lives is certainly a good intention; but no matter how many lives are saved (i.e no matter what the "potential or actual payoff"), it is never lawful to intend the death of any other person (see condition #1). Even if every last human on earth were threatened with death from agonizing disease, and if only one embryonic child's death were necessary to save all of humanity from terrible suffering and death, it would still be immoral.
Situation #2: Is it lawful to kill an unborn baby by abortion, if he/she was conceived through rape or incest?
Answer: NO. Even if the intended effect were "relief of the mother from alleged psychological distress" (which is a false hope, anyway), it is never lawful to intend the death of any other person (see condition #1), and no appeals to emotion will avail against this truth.
In my next section, I'll try to tackle the thornier issue of "to what extent can someone vote for abortion-tolerant [or tolerant of other intrinsic evils] politicians?"... which is one of the main reasons I started this 3-part series in the first place! To be continued...