Why doesn't the end justify the means?

But as young kids, we learned that the “end doesn't justify the means.” In other words, a positive outcome isn't, well, a good thing if the methods used were dishonest or harmful to others. ... On the contrary, cheating or avoiding hard classes might keep your GPA high, but using these means never justifies the end result.

Is the end justifies the means correct?

A good outcome excuses any wrongs committed to attain it. For example, He's campaigning with illegal funds on the theory that if he wins the election the end will justify the means, or The officer tricked her into admitting her guilt—the end sometimes justifies the means.