Nial was anything but fine.
Sabrina wanted to take revenge for their deceased mother. She wanted to do something he had failed miserably with.
It was only obvious for him not to be fine, but Nial couldn't tell Sabrina to not leave. He didn't dare to.
His only task had been to protect Sabrina, and killing Bella could merely be considered a secondary mission.
However, it was obvious that Bella was extremely dangerous. Her being alive would mean that he, Sabrina, and even their father were in danger.
'I should have killed her when I had the opportunity,' He thought, regretting that he had intended to torture her first.
Torturing Bella might have been satisfying and a means to decrease his guilts and regrets temporarily, but it hadn't been worth it.