The ends do not justify the means.
The golden rule - "Do unto others as you would have them do unto you"
If these two rules constitute your morals, will you be guided in every situation?
Will you ever have to do something evil in order to accomplish something good? And will you ever have to do something to another person that you would not want done to yourself? These situations occur all the time in movies, but do they ever occur in real life? The more you plan ahead, the less likely the first case will come up. People saying the ends justify the means are usually in a bandaid, kick-the-can situation.
What goal do you have that you would resort to dubious actions to achieve? When would you violate the Golden Rule?