It's symbolism. You love someone both for yourself and the other person. Sato since the beginning wondered what love really is. All that time she was keeping Shio it was for herself, she never really thought if her actions were good for Shio, at the very end she stopped looking only at herself, and thought of Shio well-being too.
well, in short she was a stupid teenager who kidnapped a girl for her own pleasures and killed a bunch of people to stay in this selfish world, she deserved nothing less than a very painful death.