While that vid of Jesy saying she was gonna punch Perrie isn't ideal, I think it's blown out of proportion a bit. I saw it at the time it happened and just thought she was being a grump and whatever and moved on. It's kinda playful language. Incredibly unprofessional given they were a)in the middle of an interview b)being filmed and c) talking over the top of Jade but I wouldn't hold too much of that against her now all these years later.
How and ever, I think the main issue is jealousy. She thought she was the dogs bollocks. Best singer, best dancer and best looking (even though she says she felt self conscious). I get it, those girls are gorgeous and I'd feel like tit standing beside them too. My first thought wouldn't be to start wearing leotards and crop tops. She thought she'd quit and off the back of the odd one out hype, shed kick ass and leave LM for dust. Without the girls, that's not going to happen.