Toward the end of parshas Kedoshim, the Torah tells us that if a man lives with an animal, not only is he put to death, the animal is also killed (Vayikra, 20, 15). Rashi explains that although the animal did nothing wrong and doesn't deserve to be put to death, nevertheless, since it caused a person's demise it must also be put to death. This being the case, it goes without saying that if a person, who could differentiate between good and bad, causes his friend to transgress an aveirah he will be punished.
Rashi continues, that in a similar vein, the Torah commanded Bnei Yisroel that when they enter Eretz Yisrael they are to destroy all the places and trees where idols were worshipped. Once again one must ask why the trees deserve a punishment. The answer is the same. They were the cause of a man's transgression, and therefore they must be destroyed. We are to learn from here that if a tree which cannot see or hear, is punished because it caused a person's sin, how much more so is this the case regarding a person who causes his friend to transgress an aveirah and stray from the path of life to the path of death.
Rav Wolbe related (Shiurei Chumash) that he was once speaking to Rav Meir Chodosh regarding the possibility of throwing certain troublemakers out of Yeshiva. Rav Chodosh answered that it might not be the troublemakers that he needs to throw out, but perhaps some of the boys that would be labeled as good boys. He explained that since everybody knows that these boys are troublemakers, no one is prone to learn from boys whom everyone looks down upon. However, there is a greater chance that they will emulate boys who learn well yet don't show up to davening, since others look up to them. The above mentioned Chazal describes quite clearly the severity of one who causes his friend to stray from the path of life.
We all find ourselves in situations and places where we are among other Jews, religious and secular, who look up to us. They scrutinize our actions and they define for themselves the proper behavior of a religious Jew. We bear the great responsibility of ensuring that no one is turned away from the path of life because of our actions. However, the opposite is also true. If one is turned toward the path of life because of our actions, we have created a Kiddush Hashem, which is such a great mitzvah that its dividends can be received only in the World to Come!