I have a theory/headcanon that Niffty offered voluntarily and happily her soul to Alastor. From her personality, I believe her sin revolves around stalking and violence and obsession toward her stalkees (see the "bad boy" obsession she shows during the series). I think when the Radio Demon displayed his power and sowed terror in hell, Niffty became obsessed with this "ultimate bad boy" and took it upon herself to find him and get to him. Once she did, in order to remain close to the Radio Demon, she offered her soul in a deal to stay by his side. Alastor probably accepted due to how entertaining Niffty could be, and being the attention seeker he is.
I also think Niffty's obsession decreased once close to Alastor, being the aroace king he is, but his acceptance of her still compels her to him.



















