crime, sin, vice, evil, hardship
辠 centers on wrongdoing and its consequences: crime, sin, vice, evil, and the hardship that follows from them.
辠 combines 自 (self) and 辛 (bitter, painful), suggesting the idea of bringing hardship upon oneself through wrongdoing. The exact historical development is uncertain.
The top 自 (self) over the bottom 辛 (bitter) paints a picture: your own actions bring bitter consequences—crime, sin, and hardship.
For サイ, imagine a sigh of regret after committing a crime: sigh -> サイ, as you face the bitter result of your own wrongdoing.