Adam Harishon was given a single commandment: Not to eat from the eitz hadaas. While at that time he did not yet have an internal yetzer hara which seduces to sin, he had to contend with an external yetzer hara, the snake, which was given the task of convincing him to transgress that single commandment. Unfortunately, the snake was successful in its endeavor. It seduced Chava, who in turn caused Adam to sin, thereby bringing death and numerous other curses upon themselves and all future generations. The snake was also not spared from Hashem's curse and among other things its legs were cut off.
Why was the snake punished? Being that it was employed by the Creator to seduce man to sin, what exactly did it do wrong? Rav Wolbe writes (Shiurei Chumash 3:14, 3:6) that it seems that it wasn't really a punishment, rather an inevitable consequence. Man was created with awesome spiritual potential. The snake attacked by enticing man that he could be even greater. If he would eat from the eitz hadaas he could even be an equal to G-d Himself i.e. he would have the ability to create worlds. When man sinned, his spiritual stature was greatly reduced. The yetzer hara had to now be tailored to fit the new "smaller" man, hence, the snake's legs were severed and its clout was greatly reduced. It would now attack man with different, lower, spiritual enticements, as the pasuk states, "And you will bite his heel" (Bereishis 3:15). The yetzer hara entices people to jealousy and anger and other lowly behavior.
When Adam failed to fulfill his mission, that mission was divided up amongst all of mankind. Each person has a certain task to fulfill during his lifetime. It is mindboggling to think about just how tremendous Adam's test was. In the single commandment that he was given he had the ability to rectify and overcome all the evil of creation. When he failed his test, his assignment was spread out over thousands of years and millions of people!
Shlomo HaMelech exhorts us in Mishlei (4:13), "Hold fast to mussar, do not let go. Guard it for it is your life." The Gr"a explains the pasuk as follows: The entire purpose of life is to rectify one's middos that have not yet been rectified. Therefore, hold tight to the study of mussar because it gives you the keys to fulfill your mission in life.
Everyone is in this world for a purpose. Each one of us has a portion of Adam's mission to accomplish and specific middos to rectify. Don't look at the person next to you because he has a totally different task than you. If you do not fulfill your job, no one else will fulfill it. Moreover, no one has the ability to fulfill it! It is a heavy responsibility but it should not be a burden. Rather, it should be our guide for life - "Does this action/behavior/lifestyle assist me in fulfilling my purpose in life or does it hinder me?" Even after Adam's sin, man has such tremendous spiritual potential. If we use it properly, we will accomplish our purpose in life and succeed in rectifying Adam's sin, thereby paving the way for Moshiach Tzidkeinu!