The end does not justify the means.

Reasons to agree:

1. I don't think you can accomplish good by doing evil.  
2. From a practical standpoint, if everyone thought the end justified the means, then the world would be a much worse place. An extremist view of the ends justifying the means would allow you to kill those who disagreed with you because
3. A lot of people have justified their actions by saying that the end justifies the means.
4. People who say that the ends justify the means, cause more problems, trying to fix them, than if they would have just stayed out of it.

Reasons to disagree:

1. If killing is wrong, would you have killed Hitler, if you knew it would have saved millions of lives? The ends may justify the means, if in the long run it helps more people than it hurts. I would have killed Hitler.
2. Sometimes the end justifies the means and sometimes it doesn't.
3. The end does justify the means when the good guys are doing the justification.