How was she killed??

When she was found out and sentenced, since she was royal, she escaped the Death Penalty, so she was imprisoned in her own castle, in a windowless room with only a small slit for food.
After several years, a servant womans curiosity got the better of her, and she peeked in the slit to see Elizabeth, who was one of the most beautiful women in europe. Instead, she saw Elizabeth on the floor, facedown, and dead.
It is unknown the exact date of her death though, since several days worth of untouched food were also in her cell.
She died after about 3 years of imprisonment,
Hope this helps, lol I know, kinda late.

Erzebet(Elizabeth) Bathory, died alone on August 21st 1614in her own castle at Csejthe. After her crimes were outed she was condemned to be bricked up in one of her castle rooms. She died 3 years after her imprisonment. It is said she died 'without a crucifix and without light'