Criminality 101 says to leave the crime scene immediately. Every minute wasted there is a minute closer to getting caught. But the Miyazawa Family killer missed that course. Instead of fleeing the scene, he made himself right at home in their home-turned-crime scene and over 20 years later, he is still getting away with it. Note: I mistakenly said Miyazama instead of Miyazawa a few times in this episode. 

Sources: Wikipedia- Setagaya Family Murder Youtube- Lazy Masquerade ABC.net Australia Unresolved