Ruka always knew she wouldn't be around long. She just didn't know she would get sick so soon. Women in her family always died in their 60's to 70's. Thats part of why she married a demon slayer. She knew about the core because her father was a demon slayer. And like her mother, she figured she would marry a man who wasnt likely to out live her. She could live as a widow for a few years. Her husband lived into his 40's, and she began to get sick. Even Shinjuro knew he was likely to die before his 60's. She would have only been without him for maybe 10 years max. She wanted to die with her husband
