Sin

Samuel said to Saul, "I am the one the LORD sent to anoint you king over his people Israel; so listen now to the message from the LORD. This is what the LORD Almighty says: 'I will punish the Amalekites for what they did to Israel when they waylaid them as they came up from Egypt. Now go, attack the Amalekites and totally destroy everything that belongs to them. Do not spare them; put to death men and women, children and infants, cattle and sheep, camels and donkeys.'" Then Saul attacked the Amalekites all the way from Havilah to Shur, to the east of Egypt. He took Agag king of the Amalekites alive, and all his people he totally destroyed with the sword. But Saul and the army spared Agag and the best of the sheep and cattle, the fat calves and lambs--everything that was good. These they were unwilling to destroy completely, but everything that was despised and weak they totally destroyed.
1 Samuel 15:1-3 and 7-9

You see, Saul just sinned in that passage. And now I will show you how his sin affected people later on because sin affects more people then just you. Here is what happened in Saul's case.

King Xerxes honored Haman son of Hammedatha, the Agagite, elevating him and giving him a seat of honor higher than that of all the other nobles.
Esther 3:1

I bet most of you have heard the story of Esther, but you might not know that Haman was related to King Agag. Esther would never have happened if King Agag had been killed.
So the brief overview of Esther is that Esther, a Jewish girl, was chosen to be the queen of a Persian king. Her uncle told Esther that all the Jews where to be killed and Esther told this to the king. Haman was hung on the gallows and all the Jews where saved. If you'd like to know more details, read the book of Esther (The whole book is only ten chapters).
Sin is a big deal. So be careful what you do because when you step out of God's will and plan, bad thing do happen to you and possibly future generations.