imo i headcanon needletail's parents didnt care about her a whole lot (hence why theyre never really mentioned in-book) and so she kinda grew up with nobody to tell her right from wrong, making her become rebellious and crave attention

when she met rain, he gave her that attention, and she loved it (and him, over time). she was blinded by the feeling of love and so she joined darktail's "kin" (also maybe didnt want to stay in her mothers clan??? idk)

but when rain died, she lost that. she lost everything she wanted and finally came to her senses, seeing all the terrible things about darktail and his group

so yeah idk thats just what i think