Rin is a revengeful spirit that doesn't know right from wrong. After killing all those people, why would she stops at Chompoo? That's the last person she wanted to torture. Rin said she saw Chompoo looking at her when she was abducted and taken away but Chompoo didn't do anything about it. Sure, Chompoo tried to do something moments later but Rin only saw that split second where Chompoo just let those people carried her away. And most importantly, Rin doesn't want Chompoo to be with Techin. She's too attached to him to let go.
Techin and Chompoo didn't die because the writer wanted Rin to exist alone. The writer said Techin tried to help Chompoo because he doesn't want Rin to kill her. He wanted to help Rin from committing more sins by saving Chompoo's life. But he wasn't able to help Chompoo so he died along with her. Because both Techin and Chompoo have their own sins that they have to pay for. As for Rin, she is too attached to her vengeance. Techin's death is the result of her sin for having killed all those people.
Techin and Chompoo didn't die because the writer wanted Rin to exist alone. The writer said Techin tried to help Chompoo because he doesn't want Rin to kill her. He wanted to help Rin from committing more sins by saving Chompoo's life. But he wasn't able to help Chompoo so he died along with her. Because both Techin and Chompoo have their own sins that they have to pay for. As for Rin, she is too attached to her vengeance. Techin's death is the result of her sin for having killed all those people.