Punishment

Punishment is an important topic in the Torah. It is a way for Hashem to show us our misdeeds and we can atone for them. It's almost like training a pet: if they do something wrong, you simply make them uncomfortable. They then have no desire to do what they had done before. In the three cases in Bereshit, the sinners were punished for doing things that defied direct commandments from Hashem. The snake caused Adam and Chava to eat from the Tree of Knowledge, so his nature was changed completely, thus making it nearly impossible for him or snakes in the future to cause people to sin. For Adam and Chava, surviving changed from "easy and G-d given" to something they actually had to work hard for. Adam had to work hard to grow food. Chava had to endure pain during childbirth. For Cain, who murdered his brother out of rage and jealousy, he was forced to wander the Earth and struggle. This is a "מדה כנגד מדה" because all murder did in a world with very few humans was leave them without companions. They had to wander the world alone. Companionship is vital to human happiness. By murdering his brother, Cain ruined any kind of friendship he might have had. Instead, he had to go through life marked by Hashem, so everyone else would know about his sin and so that they would not defy Hashem by killing him.

Basically, all of these punishments are examples of the causes and effects that might lead us to defy Hashem. It is our job to take these examples and apply them to our lives. It is important that we consider the affects of our actions and how we might better ourselves by obeying Hashem's commandments.