Dean was torn in so many wayas. He knew he'd done the right thing by protecting Sam, but because he had there were twelve more murders on his hands. He had a feeling he knew exactly what was happening. Cas, his old partner and friend, had teamed up with Meg Masters. It would explain why she was able to disappear off their radar so easily. Sam was grateful, but Dean knew he wouldn't learn his lesson. Sam was born to kill. Dean had saved thousands of people in hundreds of different ways, but he could never find a way to help Sam. His little brother was too far gone. All Dean could do was shelter him from the law.
John and Mary Winchester were killed in a fire when Sam was just six months old. Dean and he went through dozens of orphanages, but they never stayed for long. Dean was a good, normal kid, but Sam was a violent and wreckless daredevil. Some of the kids gave him nicknames like "Devil", "Freak", or "Monster". He never cared who got hurt. All he cared about was the thrill. All he craved was that rush of adrenaline pumping through him. He'd made his first kill at the young age of twelve, when another child had become notorious for bullying him. Finally one day Sam snapped. He proceeded to bash his skull in with a large rock. Dean had covered for him then, too. No matter how much of a monster Sam had become, Dean could never get him caught. He cared too much, and Sam cared too little.
Dean stayed on the straight and narrow as best he could. At nineteen he joined the Marines. He passed boot camp with flying colors, and spent three tours in Iraq and one in Afghanistan. At twenty-six, he was honorably discharged for a serious leg injury. Upon returning to the US, he joined the police force in Sioux Falls, South Dakota. The police chief, Bob Singer, took him under his wing. Bob's adopted son, Cas Novak, also grew a strong attachment to Dean. They became his first real family.
For a long time, Dean knew nothing about Sam's extracurricular activities. He would write letters to sam while ihe was overseas, but he never recieved a response. Sam had gon through two years of high school before he was expelled for continuously fighting, swearing, and threatening his teachers and peers. He was sent to a correctional facility in Sioux Falls, but all it did was make him worse, hungrier for blood. He got out when he was 22, around the same time Dean was getting back from Afghanistan. He knew about Dean's arrival to Sioux Falls, as it was plastered all over the papers. Upset about Dean's new adopted family and abandonment, Sam decided to throw his own little welcome party for his brother. He slaughtered a small family in cold blood, and wrote "welcome home" on the wall with their blood. Dean could tell immediately who had done it. As with all things Sam, he told no one.