She loved watching people suffer.
Maybe that's why she volunteered.
Every ounce of pain coming from the girl on the floor brought her more pleasure.
Every scream made her smile; every drop of blood shed made her skin shudder, in the thoughts of what she was doing.
Her heart had turned cold towards others, and pain was now something she inflicted on others only for her own enjoyment.
No one knew why she took so much pleasure in torturing others, but she did. When no one was around, she would even inflict pain on herself to receive pain.
After torturing someone, she would often take someone up to her chamber to relieve herself because she couldn't handle it.
Bellatrix knew what she was, and she took no shame in it.