The end does not justify the means.

A person should not determine his course of action solely for the purpose of the accomplishment. A person is obligated to do that which is demanded of him, and it is for Hashem to bring results. It is incumbent upon man to act, not to accomplish.

Yosef's brothers justified their cruelty to him by their belief that he was a vainglorious dreamer. This was the cause of their ability to justify their actions. We have to be sure that every action we take is correct, regardless of the anticipated result.

from the Sefer "Reb Chaim's Discourses" The shmussen of the Mirrer Rosh Yeshiva Rabbi Chaim Shmulevitz ztl page 99