Role Of Evil In Act 3 Scene 1 Macbeth

The answer to the question "why people do evil things when they know these things are evil" is ultimately dependent on perspective. So if you are wondering whether "the ends justify the means" is a sufficient answer to that question, I would argue that it depends on who you ask and the context in which you ask that person. To the people who strongly believe in the end goal, this would be considered a sufficient response because it makes them feel a little better about the evil they are doing. However, to the people who don't support the goal, this would considered an insufficient response because the means is all that those people see. Perhaps a couple of hypotheticals could help explain this.

If Armenia decided to invade Turkey tomorrow

to take back what they lost in the genocide, I would honestly be inclined to support Armenia. If people were asked me why, I would say, "to get back what is rightfully ours", but that's really just another form of the "ends justify the means" answer. Despite the fact that, in my mind, I would know that attacking turkey would be wrong, that answer would actually make me feel that attacking Turkey is okay. Deep down, "the ends justify the means" answer would just be an excuse to get what I want and it would seem to me that it is a sufficient answer.
On the other hand, if Italy similarly decided to invade most of Europe and North Africa to regain the Roman empire because it's rightfully theirs, anyone who gave me the "ends justify the means" answer would seem crazy to me. I wouldn't agree that taking back the roman empire is important enough if it meant probably killing millions of people to do so.
There is a show I watch called Arrow that I believe quite accurately represents the difference that perspective can make. In the show, Oliver Queen, a young man whose family is worth billion, is lost on an island for five years, and, upon his return, becomes a vigilante using the skills he learned while he was lost. His father had given him a list of corrupt politicians and business men and women, and Oliver, as the Arrow, goes around Starling city at night to kill these "evil" people with the hopes that he will make the city a better place. During this time, a group of powerful men and women in the city come together with the goal of rebuilding the bad parts of the city make the city a better and safer place. The only problem is, their plan is to create an artificial earthquake meant to wipe out a part of the city nicknamed "The Glades" because it is filled with drug dealers, murderers, and very poor people. They all believe that the ends justify the means, but as an audience member, I can obviously see that what they are doing is evil and they should be stopped before they kill anyone. To make a long story short, the Arrow stops their evil plan and many of the group members are arrested and taken into court. Suddenly, when the Arrow took away their ends and left them with nothing but the means, they were all filled with guilt and decided to confess in court and take responsibility for their evil actions. When there is no end to look forward to, "ends justify the means" is no longer a sufficient response and the people who were doing those evil things will realize just that.
So how does all this relate to Macbeth?

Well, an example of Macbeth using the ends to justify the means is when he decided to have Banquo killed. In Act 3 Scene 1 Macbeth realizes that, "Upon [his] head [The Weird Sisters] placed a fruitless crown and put a barren scepter in [his grip]" (3.1.66-67), and with the goal of securing the crown for his children and himself, Macbeth convinces two murderers to kill Banquo with his son. Clearly, Macbeth thinks that staying king justifies killing his best friend, however, after the deed is done and Macbeth has reached the ends by securing the crown, all that he is left with is living with the guilt that comes with using evil means. With no end goal to occupy his mind, the guilt drives him to insanity and he begins to hallucinate Banquo's Ghost. He asks his guests at the feast "Which one of you have done this?" (3.4.59), but in actuality, he is the one to blame. The "ends justify the means" can only be temporarily sufficient because those who use it are just fooling themselves. Another example would be when Lady Macbeth convinced Macbeth to kill Duncan and take his place as the King of Scotland. Her own power hungry ambitions lead her to be, "[Filled] from the crown to the toe top-full of direst cruelty" (1.5.49-50) and therefore, to become queen, she convinces Macbeth to kill Duncan. However, just as it happened with Macbeth, when she finally does become queen, the guilt fills her unconsciousness as "She rubes her hands" (5.1.28-29)

trying to wash the guilt away. When the dust settles and there is no longer an end to justify the means, the true weight of our actions can be unbearable
There are examples of people using the ends to justify the means all over the world today.

From Lance Armstrong cheating his way to winning seven Tour de France races and becoming an inspiration to cancer patients, to President Trump wanting to improve our National Security by banning immigrants of a certain race, origin, or religion, there are those that would say that the ends justify their immoral means. However, that answer is like a time bomb just waiting for the ends to come. And when the end comes, and you're left with living with what you had to do to get it, that time bomb will explode, filling your heart and mind with

guilt.