Man, what gets me about Sasori's story is how it's this slow poison, not just one big event. He wasn't born evil. It's the constant drip-drip of loss: parents killed in the Third Shinobi War, raised by his grandmother Chiyo who tried but couldn't fill that void. He turned to puppet-making, not just as a hobby, but to create something permanent, something that wouldn't leave him. The tragedy is that he succeeded too well.
He made his first human puppets out of people he cared about, like the Third Kazekage, chasing that impossible feeling of connection. But the more he replaced real people with art, the more human he felt he was losing. By the time he joins the Akatsuki, he's turned himself into a puppet, literally sealing his heart into that core. He wanted to become a timeless piece of art to escape the pain of a mortal life, but all he became was eternally, tragically lonely. That scene where he hesitates for a split second against Chiyo and Sakura wrecks me every time. The tragedy isn't just his past; it's that he built his own cage and then threw away the key.